I put one together in about 5 minutes. I bought a cooler of any size you wish. Get some heat tape and a helix or any thermostat BAM you have yourself an incubator. The most costly thing is the helix lol
A heat pad on a dimmer would also do the job and its alot cheaper than a helix. You just have to adust it until you get the desired temperatures. Be sure to do that before you get the eggs though.