I have a number that I have bought--one for turtle eggs, one for birds eggs (out on loan), a made in China reptile one and a 1960s laboratory bacterial incubator which I bought realy cheap and looks ugly as but is probably the best one. You need a low wattage heater but an accurate thermostat that will vary about half a degree each way of the temperature you want. I have tested them all and they all do that. You can buy an accurate thermostat from Homershams for a little over $100 which reads the temperature as well.
There is a firm on Trademe selling them and I think they sell a whole range of other types including 2nd hand and posibly the bits you need to make one.