You can pick up a lamp dimmer ($10): it won't regulate the heat pad at a certain degree and will fluctuate with the temps in the room, but you can monitor it with a temp gun and adjust it accordingly in the meantime.
The thermostats I get are $115, but I believe there's a Hydrofarm thermostat on Amazon that's ~$30? I don't have a link since I haven't ordered one but that's an option too.