Ontrack.com is the most well-known and probably among the most expensive. There are others that have ads in the back of the trade rags like PC Magazine, PC World, eWeek, InfoWorld, etc. Most good bigger city or college libraries have one or more of them. I know Tallahassee Public Library carried several, but some were in the reference dept. because they are expensive (eWeek, InfoWorld) .
. I can't recommend any other than Ontrack and them only by their reputation - not personal experience (knock on wood)... Perhaps others will chime in. Go join forums.anandtech.com and post there - they have a lot more traffic of techie types than here.
.bh.