I found an eBay seller offering the powder in 100gm quantities, though searches I've done suggest it needs heating to quite a high temperature (rather more than thymol) to carry out fumigation. I will need to do some more research - possibly it is effective with less heat if given more time.
Maybe I should take you up on your generous offer, Ian. Would the formalin liquid work for fumigation at nearer room temperature?
Paraformaldehyde is a polymer of formaldehyde and releases the monomer at room temperature. You don't need to heat it. Exposure will take a few days but will work.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.