Hi Neil.
Most clubs do offer handout transponders that are used on the day, they are pinned to a part of the car (such as the wing).
Personal transponders are the biggest thing out there, once you have your transponder, that's it. You keep it no matter what track you go to. It has its own unique number. There are a few on the market and here's a quick rundown:
AMBrc - These are the most commonly used at the moment, currently not available from new as they've been replaced (see below) but many do pop up for sale as second hand.
Mylaps RC - This is the new version of the AMB transponder, it's much smaller and can be purchased from
www.bbksoftware.com. Clone transponders are available if you have more than one model, that way you can have them installed in as many models as you like with the same main number.
MRT - I use these myself. Very small design like the mylaps transponder and a few options available when purchasing such as case style, led colour, lead length... MRT own a certain number of "official" AMB transponders and they install 20 numbers onto each MRT PT. The chances of you conflicting with another MRt at the same race who has the same number is a slim chance but you can simply change the number at the press of a button to have your own number. These are the cheapest option too. See
www.team-mrt.com
Hope that helps.