One rule worth considering it to take what you can see first over what you can't. If you go to Tandy, and they won't work with you, and never have anything good in stock, that is one thing. But mine has some OK leather (Canada is leather central, though I assume our Tandy comes from the same source) and being able to handle stuff is the key for me. Tandy stores are inconsistent, since they are franchises and don't seem all that centrally run. But if you have a vibrant local market, and the owner is good, the Tandy store can be a great source.
Keep in mind pros buy in volume, so they can't afford the time to pick out a hide at a time, and their supplier will work with them, and select appropriate leather if they can. So the pros source may not translate to the one holster guy. In larger towns there are often other leather sources than Tandy or the internet. In Toronto, we have several places that sell leather, and we also have a few national places, so there are lots of sources, and I am sure it is even better in the US.