I say this only so that you know it's worth searching, because I don't have any links handy at the moment: yes, you can buy the wiring or carbon inserts separately as well as some kind of controller. I say this because I've previously researched such things for making heated motorcycle clothing (which, when combined with the higher power of a motorcycle electrical system, will keep you balmy for as long as the bike is running). One can do anything from sourcing your own wire to buying pre-made carbon inserts (such as in heated auto seats), to kits you just sew into the clothing. There are pages that will tell you how to do the math for wattage draw, etc., should you need such.

It's been a number of years since I've done this research, so your options should be even more broad now, because heated clothing that isn't motorcycle/snowmobile-specific is a relatively new thing (thanks, lithium batteries!).

Anyway, the stuff is out there, you'll just have to go look for it.