Welcome @petrbow - I second what Charles says above, MuseScore can be a really useful tool, especially since once you have gone to the trouble to enter a score it's (relatively) easy to modify it to your own liking - tempo, key, actual notation and so on.  It can be initially awkward to get-your-head-around but once you accept its limitations it is a really rapid scoring/editing tool - and everything is then on your own machine.  It also creates nice looking .pdf format scores for sharing with others who don't have or need MuseScore itself.

There are sources of free sheet, including MASSIVE libraries here -https://musopen.org/sheetmusic/   - and here - http://www.imslp.org/

These of course really only have non-copyrighted, or out-of-copyright stuff.   If you're looking for more recent work - almost everything you'll want will have been ripped to you tube, or have been covered on YT - so - another way of "learning" a new-to-you piece that you like is to download it from YouTube and then use a "slow-down" program to play it back at a speed you can follow.... for instance 

http://onlinetonegenerator.com.....tcher.html

But, equally, consider Audacity as well - another free program which is a bit more flexible in its approach - you can select areas of the music, loop on that area, slow it down and so on.....  and like MuseScore, once downloaded "lives on your machine" and is more manageable and flexible than the online things (like the time-stretcher above) 

Although both free software ( MuseScore and Audacity ) they are "clean" (no threat to your machine) are used by probably millions of folks, and are maintained by a team of volunteer coders and engineers.

Best of luck with your progress !