I bought mine online second hand pretty cheap and just bought new pads. I had my baby in March and will never use a tens again. The pads stuck to everything, especially the paper on hospital beds, wires are always in the way, I accidentally pulled them off a few times, hubby couldn't massage my back without making pain worse because of pads and I don't think it actually helped anyway.
I know some women swear by them but it definitely wasn't for me.
The gas however was amazing, I used it from 8cm and it made the rest of labour a breeze!
I don't rate them for labour but hubby loves it for general back ache lol x