I can't seem to find it at Maxamps.com. I did a search for heat shrink and came up with a result for 4" tubing, but it is almost too cheap so I don't know if it is actually the right size?
M2EnergySolutions.com doesn't have a way to order the wrap by itself that I can see. Besides, it looks like the fairly thin, glossy stuff and I'd rather have the black rubbery stuff.
www.mcmaster.com looks like they have the stuff, but it's quite expensive there!
I found
this site. Still pretty expensive, but not bad. However, their order page isn't secure, so I'd have to call it in.
Thanks everyone for their suggestions!