I hired mine from a chemist in west auckland, but waitakere hospital also lend them out (although they only let you pick them up once you're in labour

), but maybe your hospital might do them, or phone around some pharmacies to see if they do?
The tens machine works by helping to release endorphins in your body, so will have no effect on the duration of labour. It can take about an hour of slowly ramping it up to really get the pain relief working. I did keep forgetting to push the boost button when a contraction came on, so it might have been more effective had i done that!
You can't use them if you want to use a bath/pool, as generally they're not waterproof, although some waterproof sticky pads could be enough to cover the wires etc if you just wanted to have a shower.
I took mine off when I hopped into the pool, and did also have some gas in the pool too. Overall, I think I'd use it again next time - don't really have a comparison as to the effectiveness as once I hopped into the pool my labour was kicked up a gear (so don't really know if part of that was taking the tens off, or just everything speeding up).