Talk to this man:

http://www.xmarksthescot.com/forum/members/artificer/

He does phenomenal custom work and just might be able to help you, so long as you don't need it next week.

Other than that, I have no idea where to point you, as Sky Highland Outfitters is the only place I've seen selling them, and of late, they seem to be having some serious...issues.

But yes, if you want a truly amazing custom sporran, talk to the Artificer.