This How-To will walk you through how to build your own Digital Dual-Stage Temperature Controller. I sourced many of the extra parts (wire, outlet) from Goodwill. The temperature control unit was purchased on eBay for $20 shipped. The project box was purchased from RadioShack for $6.50. Total cost for me $30, which is much cheaper than the Ranco and Johnson controllers sold for $150.00.
Here is the link to the Temperature Conversion Chart which includes the year round offerings from WYeast (plus quite a few of their PC strains), the temp range for each yeast, and if they recommend a diacetyl rest for the strain.