I got mine from http://www.hottie.org.uk/ who also sell replacement inner cores, which can be useful if they spring a leak. If you don't overheat them they should not leak, but if you reheat one which has cooled down a bit you only need a VERY short blast. A friend found this out the hard way - wondered why the removable cloth cover had started to feel damp.
Some other versions are filled with gel - if the hot gel leaks out it is sticky and can burn the skin before it is washed off. These from the site I mention just have an absorbent filling and water. Much safer.
From cold they just need about a minute and a half in the microwave. There is a lot of information on the site.