You may want to try your local pharmacy first. If they do not have one, and my memory serves me correctly, you can head down Pho Ngoc Khan (off of Kim Ma). I do not remember the address, but at one point there are numerous medical supply stores on the right hand side, some of which sell hot water bottles.
I suspect it's one of these:
Why they are called bottles I've no idea. More like a hot sack if anything.