Thermacare are the only ones that really work in my opinion. You peel off the backing and stick them to your skin. They gradually heat up and are amazing. But expensive. Tesco are doing 2 for 1 at the moment! The other heat pads are easier to find and cheaper but they don't go very warm and you have to stick them to a layer of clothes which has no impact really.
Why dont you buy one of those cushion type things with beads inside, that you put in the microwave? They last forever. If I see one on ebay I will let you know.
However, needing to use them for long term I found it getting too expensive so I bought an electric heat pad. Bit like a mini electric blanket.
It is fab. It has a cut out and after 90 mins will turn off, but if you leave it to cool down (which happens fairly quickly) then you can heat it up again. It has three settings and the cotton cover is washable.