Heat pads

eBay?

Any drugstore will have them for under $15.

You probably have a Walgreens nearby if you are in the US. Here's a link to a heating pad they have that is $15.99/$12.99 on sale.

http://www.walgreens.com/store/c/walgreens-12-in-x-15-in--moist-or-dry-heating-pad/ID=prod6031577-product

If you can't afford that and have a microwave, you can also put some dry rice into a pillowcase and briefly heat it in the microwave before applying it to the body. Just keep reheating it in the microwave as needed. Discard the rice afterwords since it won't be very edible.

You can make one with rice and a corner of a pillow that you can slip in the microwave, and there are others you can buy with a nice scent... cool on a cold night to curl up with too...

dollar+ stores carry them around here

they've dropped quite wonderfully in price

no special sources needed anymore