Newegg is usually the best place to buy GPUs if you live in the USA. You could try Amazon, eBay, TigerDirect, Fry's, MicroCenter, SuperBiiz, Ncix, BestBuy, Rakuten, and check PartPicker's > individual components > video cards section.
I probably forgot some, but that's the general list that I check when looking myself (+ Google searches of course).