I can't authoriatively recommend them, but I found tensmachines.co.uk which sounded good - had some machines which looked/sounded good, incl. one recommended by Midwives Assoc.
I did ask at my antenatal class which one midwives would recommend (even informally) but my midwife was totally non-commital and said she believed they all did the same job, so to go for a cheap one. I was a bit sceptical (though of course I am not experienced).
I've seen 'MamaTens' mentioned a lot by people as being good.
I'm about to order mine for a few weeks time, and this is what I've found out so far. You can actually buy them from various companies and also I've seen lots on ebay, if you find you're too late to hire (some are brand new; some you just need to buy new electrode pads for).