check alibaba.com and be prepared to buy a LOT of mats.

http://www.alibaba.com/showroom/seed-mat.html

50 each is pretty good.

look into agritape.

My son uses some for his business, and he makes them. He gets the mats, but wires them himself. I'll get to see where he gets the stuff.

Slimy Okra
I have been using old electric blankets for almost 20 years.
Shop your local thrift stores for cheapos and fold in half or whatever to fit your table/bench. Cover table with several layers of newspaper, lay blankets on paper and cover with several
layers of plastic sheeting. Place trays on plastic and you are in business. Usually low to medium thermostat settings will be sufficient. You will need a thermometer to check your soil temps.
You may need to add a top blanket/cover for germination if your work area is unheated.
I use this set-up to grow several hundred 72 cell trays each year.
Good Luck!
Bud

If I needed lots of heat pads and didn't have any I would recommend heating blankets too. Some people use Xmas lights but seems like a hassle for a larger operation. For so many trays it would be cheaper to heat the room highly. I have 8-10 mats so I have started germinating the seeds in small pots and then transplanting to cells but keeping the area warmish.

I have not been impressed with the heating mats because they seem to die after a short period of time and are really expensive. We are planning to build a heated germination table that is similar to this one:

http://www.sharedlegacyfarms.com/2012/building-a-propagation-table/

I think it would be more cost effective than the heating mats and easier to fix if part of it stops working. You can get the cables from several different places. I got the idea from this farm's facebook page. I've gotten tons of ideas from her.

https://www.facebook.com/media/set/?set=a.10151574393216934.1073741825.294625606933&type=3

Here is a link that might be useful: germination table pictures

My son got his off of ebay. I agree that the ones that you can buy from stores don't seem to last, but his does. He can get different widths and they will cut to length, then he adds the thermostat. He raises snakes and they have to be kept at a certain temp.

I don't use heat mats anymore for the same reasons you all mentioned...I used a plastic shelf heavy duty that I got from menards. Then I wrapped three sides of it with plastic, taped it on there, and the top. Then I put a bed sheet over the front to keep the heat in. I bought a little heater electric with a thermostat and sit that on the bottom shelf. That leaves me four shelves to put trays on, I can get about 9 trays per shelf x 4 shelves. I check stuff twice a day, move it to the greenhouse as soon as it sprouts. I have the shelf in the kitchen and another one in the spare bedroom. I heat my smallest greenhouse 10x20 to around 65-70 degrees to move them to at first, then to the regular greenhouse when I need more space.

Redi Heat or Agri tape...Harris Seed company has them, along with a thermostat to help regulate the heat.