Amazon's got them. So does Alibaba. They come in various lengths. Reviews on Amazon are mixed. One brand is Jump Start which is also sold on at a website called Hydrofarm.com which also sells heat mats and seedling starting kits. Prices for similar equipment seem in line with big box stores, maybe a little cheaper.
Power ratings at hydrofarm.com range from 32 Watts for the 12 foot long to 140 watts for the 50 foot. They have 110 volt, 2-prong USA-style plugs in the picture. You could plug a bunch of them into a power strip but they don't look like they can be daisy-chained. There is a user manual available for download on the hydrofarm.com site.
Amazon has another brand called Gro-Quick that looks similar.
I never tried this kind of cable so I can't recommend one over another. The built-in thermostat doesn't have any way to control the temperature so it is probably just a pre-set thermal cutoff switch to keep it from getting too hot. "Will it get warm enough?" is another question. The answer to that depends on how much soil there is, how cold it is, and other factors.
Resistance heat is used in all kinds of heating applications . These cables are just resistance wire in pre-cut lengths, sometimes with a thermistor to cut off heating at a pre-set limit. A heating mat is the same thing except the resistance wire is pre-attached to a mesh or board to keep the wire at a constant distance for even heating. With the heating cables you need to supply a separate mesh to keep the wire in position and protect the cable from digging implements. The advantage of the heating cable is that you can arrange the wire in any shape, not just the typical rectangle, square, or circle of a heat mat.
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