Hey Agentrando,
Congrats on your customs. It's quite a journey, but well worth it. Now you can create your favourite songs that no one else is interested in doing.
Your archive of 120,000 gp5s probably got shut down by some lawyers representing big publishing companies that want us all to buy all their sheet music or midis for exorbitant prices that they don't pass much or anything on to the artist at all, sometimes. Lots of these have come and gone over the years. The survivors are probably in Russia or Lybia etc.
i think the most commonly used sources on here are Songsterr and Ultimate Guitar (Russian). Take care before you pay hard earned dollars to ultimate-guitar.com for Pro membership. They have some nice "Official" gp5s but don't allow you to download them usually. Read this.
You can always google gp5 or midi files and turn up a bunch of sites of variable quality. After sorting through them they might have your song if you are very lucky. It might have 3 parts, or not. I find songs which were not hits in the USA are generally much harder to find.
Take care when buying sheet music. A lot of it is just wrong, especially with songs from before 1990. It might just be someone's idea of what is good to play on the piano.
Never heard of SmartScore. ScanScore seems pretty good. I've got Photoscore for scanning sheet music, which is also pretty good but has to be checked thoroughly for misreads of notes. It produces files in Sibelius format which then can be converted to midi. Unfortunately music xml from Sibelius is incompatible with Guitar Pro xml so it's back to midi which is not completely reliable and sometimes produces octave jumps.
So yeah, there's a lot of problems to be overcome in producing a custom DLC. As soon as you take a shortcut it backfires on you, I find. I take it pretty slow and careful.
Good luck with your CDLC.