I can't tell you where to get them, but I can tell you that they are not perfect. They're based on a mathematical algorithm that for best illustrative purposes to us lay people is "close", but not perfect". This has been written up extensively by Tim Coleman of the NE edition of the Fisherman in his recent series on wrecks.
The 3rd party programs have proved "so useless" (Tim's opinion) that he does this method instead: 1 - use the Loran that originally inputted the TD info for the waypoint, 2 - goes to that TD using the Loran, & 3 - then input that known point using the GPS.
One thing to keep in mind, GPS numbers may not be as precise as good Loran TD numbers, although they are accurate; the difference being the degree of offset to the intended bearing/waypoint. Having said that, the GPS sharpies use what is known as "position averaging". They repeatedly record the Lat/Lon position for a waypoint, then use a position avergaing software to calculate the "best fit" or most accurate fix from these repeated bearings. I don't know of any sources for this software offhand.
Remember, the DoD intentionally screws up the GPS signals - that's the cause of the variance. FWIW - you might try Tim @ the Fisherman for his opinion on the best of the conversion programs. Feel free to email me if you have any further questions.