I'm a collar addict. lol. Jackson has soooo many.
For leather, I like
Ella's Lead and
Paco Collars.
For fabric, I use
CollarMania.
My favorite tags are
FetchingTags. I've had some of mine for literally 6-7 years and they've been through swimming, beaches, mud, lakes, pools, rain, snow, etc, and they've lasted and still look perfect.
You can find a ton of super cute collars on Etsy as well.