Purchasing Transponders

[Bravo]

when we were at Batley a few weeks ago, my son used one of the club transponders. It was quite large (but he managed ok on that car).
Both he and I are building better faster cars now. Space in these cars is limited.
Jimmy (club member, from what I witnessed he is well known to you all) showed us his car, and he said he had bought and installed his own transponder. It was v much smaller (and of course lighter).
2 Q's
1). Where does one buy such a thing ? I have done a search on Modelsport and there is no such thing.
2). How does the club pc / timing system know who you are if you have one of these personal devices in your car ? When Jimmy is racing around, how do they know it's him ?
Any advice would be greatly appreciated.