for a small one, probably your best bet is to get a digital thermostat. for a heating element I would go with a replacement one for a little giant, or hovabator. I do find that the digitals are a lot more accurate, but seem to break down faster.
I personally prefer thermostatic wafer thermostats. you can easily change the switch to match the application. they dont go bad often, and about the only thing that goes bad is the wafers. I keep an extra on hand for that reason. the drawbacks - these thermostats have a slightly larger temperature range. I dont find any problems with my hatches using them.
when your incubating; keep in mind, it will never break down when its convenient. It always happens at midnight on Friday night, try getting parts then. if your using a light bulb for a heater, keep an extra handy too. the fast on/off cycling of an incubator blows light bulbs fast.
I have heard of people using water heater thermostats, I guess they work but I wouldn't recommend them. they have a much higher temperature range.