you should be able to get it at cyclocross world.com
I've used Velox JANTEX tape and it performed flawless for me. 1/2 the price of the stu thorne stuff.
The link I posted above is the Jantex as well, I've got several wheels set up using it, no problems at all.
The Thorne tape is sold at cyclocrossworld.com. Great stuff! Be careful what you wish for, though. I have a set of tubulars(road) and used the 'Belgian Method." I got a flat; even though I had a spare, I could not get that tire off - not even a tiny bit. I finally had to use a heat gun and a pair of channel locks at work. By the way, it was the front tire, and the Cafelatex sealant didn't work. The tires were Challenge Parigi Roubaix; the best tubulars out there. But unless you have a mechanic to f**k with the tires/wheels, and are prepared to walk home when you get a flat, store up a big pile of karma. Tubulars are worth the initial effort, and even I can feel the benefits. But when they go bad, unlike Mae West, they don't get better.
I have found that you HAVE to take them off annually to guaranty that they will come off. Once the glue sets up hard they are a PITA. For using sealant, the best combo I have found is Stan's in Clement PDX's. No tubes in no tubes ... ;)
I've never used tape and never rolled a tire. Just use crap loads of glue, don't bother with the little tubes of it. Just buy a can.
Having said that, a lot of people seem to like the tape so obviously it works. I'm offering up the above suggestion mostly since you are having a hard time getting the tape and someone else mentioned that it may even be on back order.
If you are using carbon rims, tape is highly recommended. If they are Aluminium then glue is fine. To remove a tire that has been taped is hard. A small screw driver can be carefully inserted under the tire after you work it loose a bit. Then you can roll the screwdriver to free the tire. Takes a bit of time, but if done carefully you will not damage the tire or rim.
Or use tire lever - takes a bit more work to get started but less risk of damage.
All the tape does is take up space and add gluing surface. If you have a wide rim and don't run sub 20# you can probably not use it. I have seen rolled tires that were taped so there is a lot to be said for just doing it right.
+1 on not using tape on aluminum rims. Layer the glue correctly and you don't need tape. More than likely your tire will fail before your glue job if you glue correctly.