I bought some old-style hot water bottles at a yard sale and used them a lot when I was having gall bladder attacks since my mother was overly-paranoid that I would burn down my apartment if I went to sleep with the heating pad on.
If you have a coffee maker at work with a hot water spigot, then you're all set for filling it up. In fact, you can usually get water hot enough out of the tap, but it depends on the temperature of the hot water heater--ours in our kitchen is scalding hot, but there's not hot water at all in the bathroom.
You can get hot water bottles at most drug stores.