[Aire valley]

Most clubs still have some form of "handout" transponders mainly for newcomers. Most racers find it far more convenient to buy their own and just leave it in the car.
The two main suppliers are Amb and MRT..... both work well. The MRT has some advantages in that it can be serviced and leads replaced. Also it is possible to get copies of the MRT, with the same number so you can run more than one car, again, without changing over the transponder.
There are often transponders for sale on here. Just make sure that you get either a "Harry" Amb or the latest MRT as these work on, just about, any club system.