The model we all use in DD is the AMB 260. Here's a link
http://www.sportstimingservices.com/...%20transponder
I'm a bit gob smacked as this is the place I bought mine from last year for £230. That site is now listing it for nearly £300. Ouch. I just sold mine for £150. Oops. Have a hunt around. As Slob says, post up on the DSC forum as I suspect there may be a few up for sale soon.
If you buy one second hand just check how strong the battery is. They cannot be replaced easily as it's a sealed unit. A new fully charged one should blink 5 times when charged, which means it's good for 5 days use before a recharge. They lose about 1 days life per year, so if it's only got a couple of blinks then don't touch it.