Karen Kay Buckely has circle bases of all sizes here:
https://www.karenkaybuckley.com/store.php?cat=2
I like this video of how to make circles using alum. foil.
https://www.youtube.com/watch?v=2INxkVfGyqE
I think the late Tom Russell made a lot of circles in his quilts & it seems like I saw a video of him showing how to make them but could not find it on the web.
I've never made applique circles but maybe some of this info. will help you.