You can get them from AMB if you want...I think the website is AMB-it.com, but often there is a region doing a group buy. It can save you a couple of bucks and help out a region with raising money.
As far as the rechargeable type goes...it's just one more thing to forget. If you have one car, just get the hardwired one, plug it into a fuse panel, and forget about it. If you have a couple of cars, and they won't ever be on the track at the same time, then you might consider a rechargeable one cause you can swap them back and forth easier, i think (I have a buddy who does this, but if you can afford a the cars he can, I don't understand why he can't spring for an extra transponder...)
Eric
NER #12