Hi John
There are two makes of transponder that I know of MRT and MyLaps links below:
http://www.team-mrt.com/
http://www.bbksoftware.com/pricelist...nent-sales.htm
The MRT has a list of stockists on that page. I have a MyLaps Harrys and bought it direct, good service arrived next day. They are more expensive than MRT but I have no complaints. I know guys using the MRTs too so its down to personal preference.
Hope this helps
Al