For holster leather, buy double shoulders.
This is the very best part of the hide, the most uniform in thickness, and has little unusable scrap.
For a typical holster a good choice is 7 to 8 ounce.
Leather is sold by an old measurement of how much a square foot weights.
For experimenting, buy the cheapest you can. For an actual using holster, buy the better grade. It'll have fewer scars or brands, wrinkles, or flabby back side.
Before cutting a holster, use poster board or cereal boxes to make patterns.
By using tape and staples you can make an experimental holster you can even wear carefully to see if it works.
After you get a good design, then cut it out and stitch it together.