I negotiated for my shamisen on ebay and received a good price. Just make sure the one you purchase doesn’t have a split skin (requires effort to repair), and I’d recommend getting one that comes with a case if possible. Mine also came with a wooden bachi which I accidentally dropped within the first week…I’d recommending having two just in case. If that’s too expensive, you could technically just use the remaining corner if one chips off.
As far as reading music goes, purchase one of those fingering strips that you can stick along the neck of your shamisen for positioning. After that, the music is pretty simple to read because it tells you what string, what position and (often) what finger to use.