I bought some magnets from here:
http://www.greenstuffworld.com/en/47-neodymium-magnetsAs of thickness you have 2 options. I used to glue the 1mm ones under the bases and on top of the movement trays first. This was actually pretty good until I found magnetic sheet metal (which was obviously much better). So if you have a metal movement tray or board I'd suggest using the 2mm thick magnets.
However...
Recently I started using different bases. They're laser-cut 3mm thick bases that are magnetic on one side and cost 0,11 pounds each. So that's 11 pounds per 100 which is fairly reasonable I'd say.
Link:
http://wargamesbuildings.co.uk/epages/9 ... ucts/mb006I use those because they match perfectly with the magnetic movement trays from the same company which makes them really lovely to make warband dioramas with.