Lots of places! I did some quick searches, Hobby Lobby for one sells individual fat quarters. But shipping costs would be an issue I think as is personnel cost from the business side so you are more likely to find bundles. Depends on how badly you want that particular fat quarter.
For online buys I usually go to equilter, check out their stash bundles too.
https://www.equilter.com/category/22/pre-cuts
https://www.equilter.com/category/38...se-scrap-packs
We all have our favorite stores, but after equilter, typically I check Amazon. Just never know what you are going to find!