Check all the usual places; Craigslist, Offerup/Letgo and the big player, FailbookMarketplace. It's worth it to have a fake account there just to access the Marketplace.
If you're looking for an actual racing frame, prepare to pay a price premium, but I prefer yard or fun karts.
If you have any racekart tracks with in a reasonable driving distance, chances are the track will have a website with a classified section you could check out and even post your own wanted ad. ~ I've had really good luck posting wanted ads at various sites for all kinds of stuff, it triggers folks to sell and eliminates competition from other potential buyers...