http://www.robotstore.com/catalog/list.asp?cid=17
http://www.ferrettronics.com/product639.shtml
http://www.rotomotion.com/prd_REV2.4.5DOFK.html
There are other sources out there as well for serial to servo circuits with Windows apps and APIs to get you started.
There was a good/simple one I came across last fall that wasn't very expensive. I will try to dig it up.