I got mine off Amazon. It came with a greenhouse kit made by Hydrofarm. There are several brands to choose from.
I'm sure more experienced growers will chime in.

-Robert

Here is a link that might be useful: Hydrofarm greenhouse kit

old waterbed heaters work great
Burpee I think makes a seed starter kit that has a warming mat in it. You can probably order from Amazon or Gemplers supply.
Undertank reptile heaters also work pretty good but can get too hot.
Tally HO!

I have 2 heat mats, one I bought from ebay an one that I got from amazon, a hydrofarm one. In my honest opinion, I feel like the cheaper one from ebay heats up a lot hotter than the one from amazon. My thermometer probe reads at 85-88F temps with it.

Ebay: http://www.ebay.com/itm/Heat-Sprout-Seedling-Heat-Mat-Germination-3-Sizes-/330495808600?pt=LH_DefaultDomain_0amp;var=&hash=item4cf315d458
Amazon: http://www.amazon.com/Hydrofarm-MT10006--19-1-2-Inch-Seedling/dp/B0001WV010/ref=sr_1_1?ie=UTF8&qid=1347468694&sr=8-1&keywords=heat+mat

I will probably get the Hydrofarm 20x20, but only 1 or 2, so thanks to all of you.

Or, maybe not! LOL

I am now looking more towards the Heat Sprout.

That one is interesting in that it can come in many different sizes. Here is is on Amazon:
http://www.amazon.com/Heat-Sprout-Seed-Germination-Heat-Dimmer/dp/B007AP2XJE

Del - I have been contemplating that one, 11x36.
2 shelves, 2 mats and they would fit perfectly.
The Hydrofarm mats would have to go on a different shelf and not fit as nicely.
But, Hydrofarm gets good reviews.

I bought two on amazon that were for window sills. I dont remember the name or brand except the box was green and they were maybe $20 each. Figuring that would be a good way to help keep young cuttings rooting over our mild winters. They didnt seem to me to produce enough heat.

This year i am going to wrap the small pot with the heat strip instead of placing the pot on top. Maybe that will improve the heat transfer.