Does anybody have a (larger) body of scoresheet images + respective
PGNs or knows which persons/entities could potentially provide those?
All FIDE rated tournaments where norms are available (so must have a minimum number of GMs, IMs and federations playing in the tournament) are required to submit PGNs of games played to FIDE to ensure the integrity of titles. This means that all such tournaments will have a stack of scoresheets which have to be entered into the computer and submitted to FIDE. This means that arbiters at such tournaments who have responsibility for inputting are likely to have large numbers of such scoresheets.
Aside: As somebody who has been the chief arbiter in a norm tournament I can tell you that such a program, if successful, would be very, very welcome.
Additionally some federations, leagues and tournaments have policies of trying to submit PGNs for FIDE rated tournaments where norms are not available. The English Chess Federation is one such and in it the 4NCL (4 nations chess league) are very good at publishing PGNs.
Entering scoresheets into the computer is often difficult and can be boring. Some people (me, for instance) are not that good at processing a stack of, say, 100 scoresheets. Other people are very good indeed. In England there is one star who is quite brilliant. That is International Arbiter and IM Jack Rudd. I don't know what he does with scoresheets after he is finished but if he doesn't throw them away he will likely have a very large pile. You will see him on the list of arbiters officiating at mega tournaments like Gibraltar and Isle of Man because he is so good at this.
To be useful your program will need to handle languages other than English, including languages which are written from right-to-left like Arabic, Hebrew and Thai.
I should add that arbiters are not particularly strict when it comes to enforcing rules around scoresheet legibility and entering the moves correctly. For instance, although the abbreviations for the pieces may be in your native language, the files must be written in English ("a" through "h") however many players use their own alphabets for these and I don't think I have ever come across an arbiter who enforces this.
Finally, here are a couple of scoresheets from a tournament where I was chief arbiter a few years ago.
Surprisingly this is one of the more legible scoresheets from this player. He did, though, use descriptive rather than algebraic notation.
Here is a more legal one (algebraic)
I wish you a lot of luck with your project but I don't think you really understand the problem. Playing chess can generate very high levels of stress in the players and this often leads to problems with how they write the moves.
PS You can download the pgn file for the tournament these games came from from the FIDE website here.