As @Patrick_Younes linked, there are some additional resources here on Bachido that will make things a bit more clear!
Shamisen divide into subtypes based on a few key characteristics, neck size, and body size. Without getting into too much detail the smallest shousun （正寸） or full size instrument is a nagauta.
The beginner’s shamisen is a nagauta made of karin wood. Nagauta shamisen have the smallest body, the highest tone, and the thinnest neck (generally 2.3 cm - 2.5cm). While typically associated with kabuki, they are also used in schools and as introductory instruments due to their relatively lower cost.
A tsugaru shamisen is a kind of futozao shamisen. Tsugaru shamisen generally have among the largest bodies, a thunkier, growlier tone; and, in modern times, the widest necks (typically >2.8 cm).
If you want more data on the difference in body sizes, feel free to click the link in my profile.
The subtype of instrument you get has little bearing on the kind of music you can play on it, but will color the sound.
Most accessories can be used with any kind of instrument, but care must be taken with neo, doukake, tenjin caps, and strings. Differing tenjin, neck, and dou sizes may preclude certain options. Strings are limited by the size of the holes in the itomaki (pegs).
As for buying the in the U.S., you’re highly limited. The Bachido store is the safest choice. Some JP shops will also ship abroad, such as Rakuya (https://shami1000rakuya.com/).
There are proxy-buyers for auctions, but I cannot comment on them. Always use care when buying from auctions.