It depends where you're ordering from to be honest. If you're getting Braid and heatshrink, the obvious answer would be to get some MDPC stuff. If you've already got some braid of some kind, then I've recently had some experience with some heatshrink I've picked up from Specialtech. I think ideally you would want some 4.8mm 2:1 Ratio, although I haven't actually had experience with the 4.8 so can't vouch for it. I have however had some experience with some 3.2mm 2:1 Ratio, which can be a bit fiddly to get to work, but it does do a pretty good job and works. Personally I might recommend buying small amounts of each size (as it's not too expensive), and seeing which works best. Whichever size works for the 24 pin should work for the 8 pin as well...
As another recommendation, would it be easier for you to just buy some BitFenix Alchemy extension cables? If you're just going to do all one colour on your braiding it may well be worth just picking up some alchemy cables, although I'm not quite sure they're UV reactive...
kd