First of all, welcome to the wonderful world of accordion! I started on B-system ~6 months ago and will share what I know.
Search “bayan” on
ruslania.com and you can find lots of books of etudes and arrangements for bayan (basically, Russian B-system). I picked up
Modern School of Button Accordion and
Music Folder for Bayan (there are 3 such books, only linked book 1 but I purchased all 3). The books there are mostly in Russian or Ukrainian, which I do not speak or read at all, but the translate app on my iPhone does a decent job of deciphering into English for me. I open the app, point my camera at the page I want to translate, and presto.
There is also a shop in Philadelphia, PA, USA called Liberty Bellows that sells all kinds of books on
their website. These include the Galliano method book (which is for C-system but I picked it up anyway and it’s still a great resource), as well as accordion-specific arrangements of all kinds of music at various levels of difficulty.
You may also want to find a teacher. I have been taking weekly lessons over webcam with Nina Tritenichenko for a few months and I have enjoyed the experience. She has
tons of sheet music for B-system, both Stradella and free bass. If you’re not interested in lessons and want to go the autodidact route, more power to you. She has a bunch of instructional videos on her
YouTube channel (tritenina) that may help you get started.
Hope that helps. Wishing you luck on your accordion journey!