Hermann Oak leather is considered by many to be the best, and you can get it at a variety of places. The pricing would be somewhat lower for you if you obtain a business license (or whatever is required for your locale). This would open up some more vendors to you, i.e., Weaver leather. There you can buy either one side or 20 sides, no minimum order required. I do quite a bit, so I buy direct from Hermann Oak. There is also leather from Wickett-Craig (
http://www.wickett-craig.com/). It's been quite some since I've looked at their leather, so I don't remember if they have a minimum order requirement. I know some top-end holster makers that use their leather and are very happy with it. I don't know of any vendors who sell Wickett-Craig leather, but there might be some.
Your customer service/reputation is going to be as important, if not more so, as the quality of your holsters. If you let the customer service go down, eventually you're not going to have many customers.
I started out on eBay because it provided some immediate exposure to our holsters to a large number of people. We also did some talking on a few of the gun forums, and in short order, we got a website up and running. Advertising costs, i.e., website banner ads, magazine ads, etc., can be pretty expensive. I always consider how many visitors/subscribers/readers there are versus the cost versus how much traffic it brings to our website. It's still a gamble. Word of mouth (by your customers) is going to be your biggest advertiser. And it will make you or break you, as it all goes back to the customer service you provide.
Best of luck to you.