Depending on your budget either a Johnson Controls or Ranco $75 prewired plus shipping (those are on/off style) or if you can splurge Helix DBS1000 $140.00 or Herpstat (don't know too much about these their are a few different models ranging from $109.00 - $319.00) which are proportional.
I have been running two zoomed 500r's for about ten months. About a four degree fluctuation as it turns off and on. This has caused no problems for my snakes. That being said, I have a johnson controls and a herpstat on my christmas list. I plan on using the zoomeds as backups.
There are alot of cheap(petstore) themostats that are made in China and are crap. Like someone just said they flucuate horribly. I now use herpstats and rancos and have never had a problem since. if it's $30-$40 from a petstore it is probably crap.
I have 2 500R's and I dont have one problem with them. I tape the thermostats temp probe directly to the heat pad and I get 2 degree fluctuations.... good enough for me!
I've heard WAY too many horror stories about the zoomed thermostats. I had one when I first got my BP and after seeing some posts and talking to people at the herp show I got rid of it.
1.0 Hypo RTB - Kotex * 1.0 Spider BP - Master Chief * 1.0 Pastel BP - Snakey * 0.3 Normal BP's - Cortana, Baby Girl & Straga