Aloha Mrgaf,
Tandy is a good place to start. I would recommend getting your hands on the book How to make Holsters by Al Stohlman. It is very good, and has the information you need for Mexican loop, etc.
You will be hand sewing, so invest in a pack of 10 needles from Tandy, plus the thick hand sewing thread they sell.
You'll need an awl to pierce the holes. I make my own awls, and would send you one as a gift of you're going to take the plunge.
For adhesive, buy some contact cement at the local hardware store.
You can cut your leather with a utility knife that holds disposable blades. You do not need to buy an expensive "round knife" or "head knife"
I save leather for last in this list, because you might need the book first before making a selection. You will be buying "veg tanned" leather. Typically you dye the outside of your holster after its has been constructed. So that blonde colored veg tan leather will do. Single and double shoulders are a nice place to start. As for thickness, you're probably looking at 6-8oz total. Sometimes I line my holsters, so I add the thickness of the lining and the outer leather and make sure it's not too thick.
Tandy is great. I am loyal to store #12 in Baldwin Park. They ship via priority mail. Their stamps leave a lot to be desired, so if you want to get carried away with carving and tooling, look elsewhere. A big vintage lot of tools from eBay can be a much better value and yield the older/better made tools.
Look me up online and give me a call if I can assist you further or mail you a hand sewing awl
Tandy does sell kits. That might be a good option. Plan on making two. Save the first holster forever just cause it's your first, and the build the second one. It will be better than the first and take less time.