A proportional is recommended for incubation. You can buy a Helix at http://www.reptilebasics.com/Thermostats-p-1-c-250.html and request overnight shipping.
The Ranco and Johnson's are on/off thermostats and people have used them, but they'll cut on and off with a 1 degree variance (ie, if you set it at 89, it shuts off when it reaches 89 and turns back on at 88).