Gunslinger sub class

so i want to use the gunslinger sub class is there a specific book that i need to buy to make it avaliable as a figher subclass

You need to supply more information than that, such as...what game system?

westmarch? not sure what you mean by game system

Hi Mectomore! Is this game using the 5e D&D rules? Westmarch is a style of play, not a game system. So far as I know, there is no official 5e gunslinger class, though Kobold Press publishes an unofficial gunslinger class in the Tome of Heroes, available in the Marketplace. If you are looking for the class specific to Critical Role, you would need to find it in some other format and input the features manually.

If you want to use a thrd-party subclass for DnD 5E, just go ahead with Keith's suggestion and copy/paste what you need in manually. Roll20 handles it really well. No need to buy any books etc. Our group does this all the time, we use it for stuff from Valda's Spire of Secrets quite a lot.