Hexamine can sometimes be difficult to buy at brick store outlets, but I have found an unusual supermarket source called Aldi's in th UK.

Bought 2 Kg of Hexamine for about $15 i.e. about 8 packets each containing 8 Tablets each about twice the size of a conventional Esbit Tab. The packaging fits quite nicely into the British Tommy cooker just like bacpacjac's photo.

I have vacuum sealed the packets as a last ditch fuel source.