Most times the old pc cooling fans are salvaged from old computers, I suppose that you could find one at a computer repair shop if you don't have an old pc around, or buy a new one at a computer supply, they shouldn't be expensive, I have seen them for about $5 online.
Go to Radio Shack and see what they have. You need to have some sort of spacer between the fan and the wall of your incubator to allow for proper air flow. You could use long machine screws to attach the fan. Before you attach it to the wall, add a couple of nuts and then a washer behind the fan and then run the screw through your bator wall and add a washer and finally a nut. That should be adequate for spacing.
The one we bought from Radio Shack was around 20 dollars. It was heavy duty and I belive 110 volts. That way we were able to wire it in with the thermostat and light and didn't need a separate adapter.
You can work to rig a single pole, lower water heater thermostat - as several of the "how to" homemade incubators use - those will run you 8-11 dollars but the do have a wide variance and getting them in the right place and adjusted properly takes some work.
Or you can go to a reptile supple site and pick up the Reptitherm 500 or better and use one of those and they run you 25-30 but they have a very small temp variance. You then wire your choice of heating element/s and just plug it into the Reptitherm.
Or you can buy an INCUBATOR thermostat and element... Which, you have to figure is designed to work in incubators - but will run you 20-25-30 depending where and shipping.
Do not plug fans/ or wire them into the thermostat set-up. Fans should run constantly regardless of temps.
And 110 volt fans if you can afford them are worth it long run.
You can go as cheap as computer fan, light bulb and water heater thermostat and some tweaking will get you a bator that hatches eggs.
You can start piling on the things that make life easier - better thermostats, different elements, 110 fans, etc. as you go along. Which is what I did. Started basic, learned a LOT, saved some money and tweaked what I wanted to.
Everything about it is up to you - the basics can be made to work. You learn a LOT tweaking your own.
Danny will come on talking about water heater variances being wider than is practical - while true at the outset, you can learn to set them so the swing works if that's all that's in the budget and he doesn't seem to understand at all the difference between 8 dollars and 20-25 and 30.
I certainly do. If I can hatch with an 8 dollar thermostat and a 6 dollar light bulb and socket, and a four dollar fan, then I'm HATCHING at the price of his thermostat.
We wired our 110 fan in with the bulb and thermostat but put it in line before the thermostat so that it ran continuously while the bulb turned off and on. I was really pleased with the incubator thermostat we used (like at Meyer's). Just remember. Calibrate your thermostat with an empty bator.