I just applied for a replacement title in Maryland. Luckily for me, the title originated in MD, so it was an easy process. However, I would think the search method would be basically the same in any state. They were able to find the original title number by cross referencing the VIN #. Then we were able to request a replacement title with the title number. If you know what state the car was last titled in, you go go through this process, however you would need the co-operation of the last owner of record.
As far as the online title services, I think you technically sell the car to them, they obtain a title in a state the honors a bill of sale, and then they resell the car to you with a good title. I think it's rather expensive, but I don't really know, I've never used their services. They were my next option if the Maryland title search didn't work.
Good Luck,
Larry