The most economical way to purchase leather is by the side. It costs much less per square (one square foot) in this quantity, because you purchase what would otherwise be scrap. If you plan to make a lot of leather gear, I definitely suggest you look at this approach.

However, I'm well aware that some people either:
A - won't necessarily make a lot of gear, or aren't sure that they will, or
B - don't want the upfront expense (~$150 or so) for a full side of leather.

With this in mind, WWL Hardware offers a selection of precut strap. This is heavy (8-10 ounce) black latigo leather with an attractive, subtle top finish. It's an Indian tan with the characteristic yellow center, so plan on dying your edges.

To keep things simple, I carry a few stock lengths. I picked these after a comprehensive analysis of the WWL projects, and hopefully they closely match what you'll need for yours. I chose 2-1/2" for the wider offering, but you can certainly cut it down with a strap cutter or by hand for narrower base straps.