I had these "special" templates cut at the local Ace Hardware store.
Caution: It is hard for them to cut acrylic (or whatever they use) to exact size. The squares can be sanded down to get closer to what you want.
Also: Some have much more cooperative people than others. And more precise.
They weren't exactly cheap, but I thought they were worth it.
I also think I saw some sets available at Keepsake (?). I got hooked on the idea when I had purchased a set of them, but I also wanted the "missing" ones. I think the original set was 2.5, 3.5, 4.5, 5.5 and maybe 6.5 inches.
The reason for the other ones was Amy Simms book of I Spy type quilts - and she had these 5 inch templates that were "oversized" for 4.5 inch cut squares. Well, I wasn't going to waste all that fabric in those 5 inch squares that I had cut!