Same as nannyl, l got a femme tens from ebay, unused, for less than £20 including postage with the thought that I could use it again and possibly resell in the future and make some money back so I would be quids in!
You need to keep an eye on a few though, some go for heaps and check if they have unused electrodes or not as otherwise you'd need to pay for these on top.
I got mine from here £23 for 7 week's hire. Came in two days, but haven't used it yet as I'm still waiting for this baby to shift her ass and arrive <not bored or bitter emoticon> it was the one my mw was recommending.
I hired mine from Boots. I did the job really well, 9 pound baby and no other pain relief. Customer service was really good, if you go overdue just ring them and you get to keep it longer at no extra cost. However wish I had bought it the first time round, I am on baby number two now and need it again, the hire/buy price difference wasnt all that much.