Take a look at grainger. This is the Ranco single stage model...unwired. All you need is a cheap extension cord from HD or Lowes to wire it (diagram included in instructions). I chose it over the Johnson model simply because its digital.
Finding a cheap t-stat has been my quest just recently. If you are not afraid of some basic wiring here is what I came up with:
I used a mechincal t-stat out of a room a/c unit that I had aquired second hand for $1. I wired this in place of the original unit but had to mount it on the access panel of the freezer as it would not fit the original location. The probe was larger than the original and was a tight squeeze going into the original location but I was still was able to get it in there. Prior to installing, I had to recalibrate the units temp. range before installing. Even if one had to buy it from a appliance repair place, I would have to think it would be much cheaper than a prefab. external unit. The only drawback is that once you reaching freezing temps., it locks it in the closed (on) position and you have to unplug the feezer to thaw it out. There are other obstacles using a chest freezer but that would be another thread. As much as I would like to take credit for this, I am sure it has been covered before so it might be of interest to do a search for further details.
I have the original one you listed (looks like the Johnson Controls model) and there is no wireing to do just plug it in and go. It keeps the temp good with in say 2 degrees of the setting (I live in Florida and the temp flux alot in the garage) it is in an old chest freeze, but works great with little no fuss.