This year I tried using a waterbed heater for my seed starting and I don't recomend it. I used an old air mattress under it to protect the table and the heater mat was still too hot. I put it on the lowest setting and it was too low. I turned it up but it overheated and burnt a huge place in the middle of my table and melted the plastic starter trays I was using. I'm lucky I didn't burn the house down. I tried using a heating pad before and that didnt work either. Now I'm using insulated disposable coolers with the potting mix and just a grow light and am doing better. It's possible the waterbed heater was defective as I've seen where others have used them and done well.

I buy everything that I need from Charlie's greenhouses they are located just outside of seattle wa, my heating cables were bought from there and I have used them for years, also they have great prices on greenhouse equipment.
Nancy

For just about any GH accessories, including heating cables, try ACF Greenhouses, Farmtek or IGC. All carry a huge selection and for virtually any item, will be a lot less expensive than Charlie's.

But to be sure, you still can't beat just googling for "Soil Heating Cables." You never know what other vendors may be out there, hungry for your business.