Home Depot, Lowe's, Ace Hardware. Tractor Supply also has it but only inseason.
That you buy at the hardware stores will say not intended for anything other than rigid pipe. That's the tape most of us use. An rv park in Golden, CO, makes the hoses up with heat tape applied and this is what they told me. Run that tape parallel to the water hose, taping it in place every couple of feet. If the tape is longer than the hose, double it back on the other side. Make sure the tape's thermostat is outside where it will not be covered up. Next, cover the hose with foam tubing intended for water pipes. Available at hardware stores and even many Walmarts. Tape (I use duct tape) this foam sufficiently so that the seams do not part and allow access of cold air to the hose. On one of mine I even taped the entire hose but that's not necessary. You can just tape it every foot or so, just so it is tightly sealed.
Tractor Supply has heat tapes that can be wrapped around a pipe or hose. That will work, also, but a longer heat tape would be required. I use this type for heat taping the park's pipe (the above ground park) and spigot.
Once the taped hose is connected to the spigot and the Montana, I wrap roll insulation around the end points and tape them in place. Likewise on the park's spigot. That part not heat taped I wrap with insulation and duct tape it. If you have the inside convenience center, I do not insulate it there. There is plenty of heat from the furnace in there. I do, however, plug the bottom hole with rags. I do that to help keep critters out, anyhow, but also the cold.
We've been down to five below zero (F) without freezing a hose or park's pipe.
It's kinda hard to tell from this picture but the cable is flat.
In our current connection you can see how the heat taped hose looks. In this park, the park's water connection is inside that box and is covered with insulation. That's sufficient for the temperatures we get here. The roll insulation where the hose goes into the box is covering the connection to the hose that's inside the box and also blocking cold air from easily going through that opening. 22F last night and we had external running water all night. The extra hose laying there with the black tape on the end is connected to the black tank flush.
