I don't have a link but I've always had good experiences with the type of heating pad used for sprains and that (on low setting). I got mine from Walmart years ago. I've also used a standard 60W bulb under my tray pointing upward. They provided enough heat for me to get lettuce growing.
I used that DIY heat pad posted above last year. It worked great! I would definitely get a thermometer/automatic shut off as it gets really hot (mine would get the soil up to 85 F even though the house was 55F).
An old fridge with a 40 watt bulb works really well. One of the most pro farmers I've ever seen had two fridges hooked up like this. Just the tiny amount of warmth and light is enough to get them to pop up and as soon as they start flashing the cotyledons she'd move them out and put them under a tented table within the greenhouse. This table had an inch or so sand layer on it which I guess acted as thermal mass. She'd have a 100 trays popping at a time. super slick system from start to finish that one.
Freakin' hippies and Squares, since 1986
Location: Currently in Lake Stevens, WA. Home in Spokane
Years ago, I had just been laid off, and as a single parent realized that I needed to grow some of our own food, even in an apartment with no sun or soil. I scored a plot in a Pea Patch a few blocks away, and bought some seeds. I realized that I needed a head start getting seeds started, so I went to the local garden store and was shocked that it would be $40 for a heat mat...couldn't justify that on my budget. On the way home, empty handed, I stopped at a yard sale. I ended up buying a crock pot with "High/Low/Stay Warm" settings for a buck or two. Worked perfectly on the stay warm setting for my plants that needed a warm soil for germination. We ate free tomatoes/peppers until November.
I can't take it! You are too smart for me! Here is the tiny ad: