Assuming you're talking about original art I think Etsy is your best bet. Sometimes eBay has some. You also get some really cool altered art where they extend the background etc. Its not cost effective though and there's a lot of ai art now. My rule of thumb is anything over 4$ gets proxied now for a deck. There are sites like https://tolarianlibrary.com/ that do pretty good 1:1 copies that pass the sniff test for casual play. I really really hate when art is economically locked, philosophically art should be for all. I try and always proxy the prettiest version of cards. The secret lair copies aren't perfect but their pretty darn close. There's a handful of other sites I can DM you that I trust. To answer your original question I think there is a site for proxy designs, but they don't give you the cards just the designs.