I agree that each "charm" should be unique.There are multiple ways to get those unique fabrics without breaking the bank. A site that sells fat quarters of I spy fabric, is
4mt3boyz.com. Another way is to join an I Spy swap group. A third way is to use borders between rows.
I joined a swap group several years ago, but acquired enough squares to make many, many quilts, which I'm now sharing at my guild. The group has has a facebook page (
https://www.facebook.com/groups/1858499157698930/). The logistics of the swaps are handled at groups.io. (That's where many of the old yahoo groups went.) They do either 10 or 11 swaps per year. When I was active the rule was quilt shop quality fabrics only. They usually do one pieced swap per year. (I've not participated in a few years, so rules may have changed.)
I've made many I spy quilts. I'll post the ones I like best.
bkay
I'll have to post later, as the files were too big.