Your local Boots will hire them out, for about £25 I think. I think I got it about a week or two before my EDD so that I could practise using it, DD was 10 days late so I was a bit late returning it - officially I think you have it for 4 weeks - but they were fine about it. I found that it really did help with pain relief.
i got mine from here - looked on here for advice on the best TENS machine and a lot of people were recommending the Elle TENS, this was the cheapest place to hire it that i could find. £25.99 for 30 days, arrives 3 weeks before EDD.
I went to this site who were v good. You just tell them your EDD and they'll send it out in time. They also enclose an SAE so you can just chuck it back in the post. Good luck! I wasn't convinced mine was helping until they took it away from me (prior to c-section, but that's another story) and I could certainly tell the difference!
If you're bothered about hiring and having to keep it longer than 'allowed', you can buy one from LLoyds Pharmacy for only £29.99! You can then keep it as long as you like (I'll hold on to mine just in case I crazily decide to have another - first baby due in a couple of weeks) or you can ultimately sell it on ebay.
I couldn't find any online to buy at this price - all those listed were at least twice or three times that price. However I saw Lloyds advertising in a free mum-to-be mag I'd been given by my midwife, so thought why not?