If your lucky enough to live near a garment district like in LA or NY, you can buy in person.
Much of the flea market stuff is bough by the crate from liquidation outlets where department store chains, etc. dump their customer returns and last years designs. Try a search for: liquidation outlets.
Every state and city has a license requirement. Generally, if your sales are over a certain amount, annually (usually very small, like $500), then you need a business license. Reselling, itself, is not illegal' as long as the merchandise is not illegal knock-offs.
I have no experience in selling or business, but since anyone can buy wholesale clothing online, is it illegal to buy it and then re-sell it on Instagram for example? I figured that's how people at the swap meets and flea markets do it, or am I totally wrong ? Or how would I obtain a license to do it?