I've purchase a lot of leather over the years, and agree with Tracy in that is can be frustrating. It's another entire aspect to learn. What I can say is that all leather IS NOT created equal.
Buying the cheapest you can find, will only cause you grief, and lessen the quality of the end product.
Chrome tanned leather.....NO! Ever see an old knife with green goop all over the guard/hardware? That's because the sheath was made of chrome tanned. It's the cheapest method of tanning, but the down side is that all the chromatic acids can never be rinsed out, and are reactive with various metals/hardware. Chrome tanned leather are also much more difficult to bend, mold, etc.
Oak/Veg or other naturally tanned leathers: YES
I agree with Vonbrown on Herman Oaks, Wickett and Craig, as well as buying more at a time...... I've tried a number of times to mail order leather, but more often then not, it gets returned, even though I stress that I only want the BEST they can provide.....then the leather shows up with 25% or more unusable due to scars, brands, cuts, or tears. Now, unless the store has proven they can meet my demands, it's buy only in person.
Right now, the only outfit I trust enough to mail order from is Wickett and Craig. They don't offer 8/9oz...... they offer either 8oz or 9oz, and will skive exactly what weight I ask for. I've also never had to return anything from them. I tend to buy either a double bend, or a double shoulder....generally comes to 14-22 square feet of leather. As long as you keep it in a temp controlled environment, and keep the critters out of it, it won't go bad on the shelf.