Someone here has 5" sq I Spy swaps, which are great! I use my Fiskars fuse with an Accuquilt die to cut 5" squares to trade, and then use the Accuquilt hex die in the Fuse to cut them into the hex shape.
Ebay also has lots of people selling 5" or 6" squares specifically for I Spy. Some are dedicated boy or girl squares. You have to pay attention in that some of them you get quite a few more squares for the same price as someone else selling a fewer number.
There are also Yahoo groups that do I Spy swaps, but I don't like them as well. They're usually too specific for me.
You can also purchase I Spy squares from on-line fabric shops...Etsy among them. Just google I Spy squares.
It's a lot of fun! Hope you enjoy the collection process!