He produces a nice looking radio URX-1 - bit expensive (but can I manufacture them? Maybe not for very much cheaper, though my time isn't valuable!). Only issue is that firmware isn't open.
Find a bit more foxhunting information here, including Australian scanner, which might be useful for a rotating DF, but otherwise probably not great.
Hamtronics produce board-level VHF receivers, though he notes that some of the good features he took advantage of have changed. USD 229.
Bob points me to Encounternet, run by Jon Burt, a PhD associate at WSU.
Looks a bit like the Markham system, though they managed to get 1 gram transmitters, and eveything will be released under GPL around 2011. Anticipate tag cost of USD 100-200. Smart transceiever tags.