You can find some resources on the web. Generally, you will be limited to traditional pieces for free. Their arrangements will not always match your expectations.
For free options, look here:
https://shamisen.space/
http://www.mu-tech.co.jp/music_files/shamisen/index.html
For free and paid, look here:
http://shamimaster.com/gakufu.html
For paid and downloadable, look here:
You can find books online at various stores. Some of which may require you to import from Japan.
Rakuya has a number of books available. Some are focused on traditional pieces, others have modern ones too.