I'm working on a lot of older bikes recently, and I never seem to have enough rim strips in the proper size. I've used quality gaffer's tape for rim tape successfully, but grew weary of cutting or tearing it into the narrow widths needed. So I just bought a bunch of rolls of the stuff in 1/2" (13 mm) and 3/4" (18 mm) widths in 180 ft lengths, so each roll should do about 25 rims.
And I know that buying this stuff in colors is pointless because it's going to spend its life buried under an inner tube, but hey, they were all the same price, so why not. And I got all of this for about the same price as 7-8 rolls of specialty cloth rim tape like Velox or Zefal.