Where is the cheapest place to get a microwaveable heat pad from? Thinking of getting one for my girls & maybe the buns too, but want to make sure I get them at the best price 

Amazon...!
£14.50 and supersave delivery...
http://www.amazon.co.uk/gp/aw/d/B0014LJKUA/ref=mp_s_a_1?qid=1326574525&sr=8-1
x
That's where I got mine & it came super quick![]()
I have been told by breeders that Guinea Pigs dont need heat, so long as they are not in damp or draught.....I actually have a heat mat which plugs in and i placed it under the indoor cage (which is in unheated conservatory)but was told not necessary!