A couple of sites I've used for I Spy fabric are
4my3boyz.com. She offers fat quarters on most of her fabrics. She gives you a description of how large the print is, so it's easy to tell if it will work in the size square you are cutting. She usually has lots of fabrics that are usable for an I Spy swap. Each section (animal, food, sports, etc.) has some sale fabrics as well as the $1.00 fat quarter sale on the main page.
I've also used
marshalldrygoods.com. They have lots of good fabrics (Name brand, as well as their own creations), but have a minimum purchase if 1 yard of each fabric. I was able to use it when I was active in the I Spy swap group that swapped monthly. (They've transitioned to a Facebook group - I don't do Facebook.)
bkay