I had some very small ones in white plastic which I covered for a chiffon blouse. Basically you gathered the circle so it pulled up inside the front cover and then pushed the little back in by hand. I'll go and see if I still have the packet with the name.
Uh oh, I have two, Coats and Newey - both probably out of business now. Sorry.
The packet shows someone using the end of an old cotton reel as a tool to push the back on. I wonder if it would work with the ones you are buying?