I’m gonna recommend the usual trio of eBay, GOAT, and StockX (though I would look at StockX last) to find sneakers from private sellers. Sometimes good deals can pop up on all three platforms. There’s not a lot of Yeezys anymore on online retail stores at the moment but I do see a few 350s on DTLR and Hibbett right now.
The 350v2 and 700s are always a favorite. I would avoid the CMPCT version of the 350 and the MNVN version of the 700. They’re perfectly fine shoes but they are generally less liked when compared to their OG models.
I use the usual two other platforms but I’ll use StockX if the final price ends up being lower than the other two or if I can’t find any listings.