Kiltmakers themselves typically don't have pieces left over. Tartan is so expensive that we order only what the client needs to make the kilt. All that I have left after having made a kilt and a set of flashes are what I didn't use for the top band (typically a yard by maybe 4-5 inches) and the arch-shaped pieces from cutting out the pleats. I usually send these scraps to the client along with the kilt.

Weavers of tartan, on the other hand, commonly have remnants. You can check Fraser and Kirkbright or the Scottish Weaver:

http://www.wooltartan.com/tartans5.htm
http://www.thescottishweaver.com/tartanremnants.html

Just be careful that you check the fiber content (not all of these remnants are wool) and the weight before you order.