I actually JUST got a TL-SR21 and had it converted to a track whip (it's originally a 8/9/10 speed cassette whip). It works really nice, it's just like the SR22 but has a couple links of chain where the 22 has that curve to do the function as a chain. Hard for me to explain without pics...
but yeah, reason I didn't get the 22 was because I was in a similar situation as you. All shops were out, and the distributor said they weren't getting any more.
Hold out for the 21 or 22, they're totally more worth it than buying the two tools seperately and the 21 even comes with a magnet at the last link of chain to keep it tied down when you're using the lockring side. Real nice addition.