Cheeky monkey carries the sitz bath crystals. 590 Oxford Street East (at Adelaide) or 555 Wellington Rd. (b/w the Lamplighter and Commissioners). I'm not sure if they'll have them in stock for you but they're definitely listed on their website.

Personally when I had my daughter the midwife froze some pads for me. Apparently you buy food grade aloe vera gel/juice (health food store) and witch hazel. Then you mixed in a spray bottle, and then spray the pad until they are well soaked, but not dripping. Then freeze them in tin foil. Hope this helps.