Well... a reliable thermometer and hygrometer would be my first choice.
Are you asking where to get these? Also, my question is, whats your budget?
For the source of these, I recommend
ebay. I got a digital cigar humidor hygrometer off
ebay, made to protect expensive cigars. It tends to be reliable for that reason. Mine cost about $15. There are also cheaper analog versions available.
For a thermometer, you can use the inexpensive infant thermometers found at any discount store. The range of temps for humans is the same range for incubating eggs, so this works.
They are also guaranteed to be accurate to within .2 degrees... more than enough for an incubator.
These are not fancy, nor purpose designed, so they can be fiddly to use. This is especially so for the thermometer. A good substitute is an aquarium thermometer, with remote probe. These are also found on
ebay. I place the probe in a water weasel and the controls outside the incubator chamber, where they are accessible. They are also more than accurate enough for the purpose.
The two together will not likely cost more than $20-$25 and I used them successfully when I was incubating, back in the day.