I bought mine from ASK Direct, in Tottenham Court Road. (
http://www.askdirect.co.uk, I think), but do read the thread on RW for more discussion about where and whether to buy.
The bad news this morning is that I think the thing has expired completely. I've recharged it but it won't switch on.
I've called ASK and they've said they'll replace it, whch I'll aim to do tomorrow.
Despite this setback, I'd still thoroughly recommend the gadget, or indeed the Timex SDM which I also have. The Garmin has more features than the Timex, and doesn't have the separate arm-worn transceiver, but essentially they do the same thing.