When buying on-line it really comes down to who has what you are looking for.
As stated R&M Custom is a good place to look as is Arizona Custom Knives. I would also add Plaza Cutlery and Nordic Knives.
If you don't have a certain knife in mind all of these carry a wide range of knives in price and quality.
i can't tell you how many times I set out to hunt down & buy a certain custom knife,only to bag a different one .