garden centres, brew shops and pet centres sell small plastic heat pads for about $40, we use one for my bf's brew yeast. We got it from oderings. It is designed to keep things about 20 degrees or so. I wish I could find one that would be just a wee bit hotter then it would be mint for putting small betta tanks on! Yes I did try putting a small empty (other than water) tank on it and taking the temperature and it could only get it to about 18 degrees consistently, thats not quite enough for a betta boy right guys?
NB I think you may be able to order one online from oderings but then you will get stung for a courier. Anyway, they are about 25cm square, royal blue and waterproof and bloody fantastic if you ask me. I use them for all sorts of things such as warming up sick animals, germinating seeds.