Hello Alan and welcome to the forum.
Heat mats and suchlike can be bought from all homebrew shops and homebrew internet retailers. There is a bit of a debate as to whether the demijohn electric heat bands are any good though. They are not thermostatically controlled and tend to cook the brew because the temperature is too high.
I have never bothered with the mats or belts, mainly because of the price. The alternative I always use is a thermostatically controlled electric blanket. I bought a single one from Argos and at the time it was about £12. It has three heat settings and I wrap it around my fermenter using a couple of bungee straps. It covers the whole fermenter and can be washed when required.
Just this morning I wrapped it around my bin of fiery ginger beer because it was minus three overnight. Once the brew is warmed up a little I will switch it down to number one and this will ferment comfortably at around 24 degrees.
Cheers Neil :thumb:
