Personally, I think books are the way to go for making glazes because it's basically like learning how to bake from recipe books. Buy the ingredients, weigh the ingredients, mix the ingredients - it's way way cheaper than buying pre-made ones. It can of course get a lot more complicated when designing your own base glazes, but I haven't got there yet. My favourite glaze books are:
- Mastering Cone 6 Glazes by John Hesselberth and Ron Roy
- The Complete Guide to Mid-range Glazes by John Britt
Glazy.org is a good resource, but it can be tricky to know if the recipes create a stable glaze when they're user-submitted. Personally, I'd rather go with recipes from an established glaze specialist, particularly for functional ware.
Below are a few videos that I found helpful for glaze testing:
Jonthepotter - Throwing Test Tiles
John Britt - Colour Blend
John Britt - Triaxial Glaze Blend
Old Forge Creations - Waxing and Glazing Mugs