Weather in Mar - disposable heat packs?

Hi, I'll be heading to tokyo this Fri and judging from the weather forecast, it's going to be pretty cold. I would like to find out where can I buy disposable heat packs (those that adhere to clothing)in tokyo? Is there a japanese name to the heat packs so that I can express myself easily? Or any common brands?
Thanks!

It's called kairo and you can find it in drugstores, supermarkets and convenient stores

I suppose it depends what you mean by cold.

This Friday is forecast to be 20 degrees, which is warm for ths time of year. The weather has been very varible for February, with some day only 5 or 6 and others up to 18 degrees.

To be precise, ask for "tsukai-sute (disposable) kairo (poratable warmers)" although nowadays, kairo mostly means disposable. Each disposable kairo lasts for about 5 hours or so.

There are various kinds. Some for your pockets, some to paste on your back, some to put in your shoes, some to stick on your socks in case you take off your shoes. Just be prepared and communicate with gesture.

Have fun!