A lot of our modern fabrics are oil-derivatives and if we have a true oil crisis, those fabrics will become increasingly expensive. Natural fibers will still be around, though, so it's not like we'll be naked.
And that's a Good Thing.
Clothes themselves won’t be hard to find. We are inundated with all kinds of clothes now at flea markets, garage sales, resale shops, and just thrown away. Fibers and fabrics are everywhere, and if we gathered it all up, we’d have enough to clothe every being on the planet a dozen times over.
The problem is, most of these clothes are frippery, fleeting fashions, meant to last a single season. Some are just ugly and others are fragile. Most synthetic fibers don’t keep people warm in cold weather or cool in hot weather, and indeed, often do the reverse. A lot of them don’t wear well without special treatment – fading, pilling, sagging. The worst part is that many of them are so flimsy they don't last at all - paying $80 for a blouse that can't make it intact through a single laundering is ridiculous. You are quickly dated wearing modern ready-made fashions.
Natural fibers like cotton, wool, linen, bamboo, silk, palm leaf, silk charmeuse, silk chiffon, wool challis, cotton challis, wood-based rayons, raffia, mohair, jute, cashmere, angora, alpaca, ramie, sisal, and hemp wear well, are easy to care for, and continue to look good years after they are made. Just look in any museum at the clothes and uniforms that have lasted centuries and were hard and well-worn before they were put in a museum.
The goal in preparing for a modern recession/depression is to select clothes that are of a cut that will withstand the vagaries of fashion and made of a fabric that can handle being hand-laundered and line-dried. If you add these clothes to your wardrobe a piece at a time now, when you can afford it, you'll be prepared when harder financial times arrive (if they do). and even if they don't, you'll have a nice core of solid, dependable clothes.
Now, why does it matter that you have classic clothes that wear well? If you have a job, especially an office job, you will be judged on your clothes and looks – and in a recession when jobs are scarce and companies are just aching to find excuses to reduce their workforce, anything that helps you keep your job is important even something as trivial as clothes. If you continually look well-kempt, in clothes that always look professional, you will be actively working to keep that job. Modern fabrics may look really good for a while, but they don’t bear up to repeated washings. They pill, shred, fray, and weaken, and it’s hard to make them look good again. It can be done but it takes skill and time to do it. I’m assuming you have neither the skill, the time, nor the desire to spend that much effort on repairing and refurbishing your clothes.
Select cuts that have remained in fashion for decades because chances are, they’ll continue to remain in fashion. Go for semi-tailored looks in plain colors – browns, grays, blacks, and dark blues fit anywhere and anytime and look good on most people. Because of weight shifts, don’t select fully tailored clothes unless you can re-fit them at need (or know someone who can). Semi-tailored clothes in a larger size than you normally wear and taken in (with the excess fabric kept in case it needs to be let out later) is what I’d choose, but this is entirely up to you. Avoid frills, ruffles, prints, and patterns. You can embellish your plain classic clothes with accessories (ties, scarves, ascots, boutonnieres, lapel pins and brooches, tie pins or clasps, scarf pins, belts, bracelets, armbands, purses, bags, shoes, hats, hairstyles, pocket plackets, removable trims…), but the basic clothes beneath should be flexible. Think of all of your clothes as "the little black dress" that goes anywhere, anytime.
Having good clothes you can wear to work can make the difference between staying employed and looking for work in a glutted job market. If you do need to look for work, having good clothes can be the edge between getting and not getting a job. Unless you own your business, there is no guarantee you’ll keep your job if times get tough. Any edge that helps you keep your job or find a new one is going to be important. If you already have a classic professional wardrobe before the recession hits, that’s one less expense you’ll have. And if the recession/depression doesn't worsen, you’ll still have a nice wardrobe you can wear mingled in with your more frivolous and fashionable things.
Make sure you have at least one pair of really good quality shoes you can wear with your classic clothes. A good pair of shoes should last you years (mine have lasted me nearly 40 years and they still look almost as good as the day I bought them - they've had to be re-soled once in all that time) and will look good, too. Take really good care of these shoes and only wear them for work. Protect them by wearing cheaper shoes to and from work and when you’re not at work. This is especially critical if you have an odd shoe size (anything over a size 9 or under a size 6 or extra wide or extra narrow). You can also protect them by wearing over-galoshes.
You may also want to set aside some specialty clothes and shoes: durable protective clothingfor heavy work (yard work, carpentry, construction and building, sewer work, welding, or any other work that requires protection while you do it), protective clothing in case of an epidemic, barrier clothing in case of chemical exposure, and, of course, formal or fun clothes in case of formal events or parties. We'll still have formal events and parties, no matter how bad things get, even if I'm the only one hosting them.
The point is to invest in a few clothes that are made of durable, long-lasting fabrics in neutral colors and classic cuts so they can be worn for years, if necessary.
What you wear outside of work is your choice. There will be lots of recycled clothes available so you will have a goodly selection of casual clothes to wear, and lots of clothes to take apart and repurpose. In fact, I can see where sewing skills and a warped sense of fashion flare would be very profitable during a recession/depression.