If you really want a paper Book of Mormon but want to avoid interacting with people who will try to convert you I recommend checking a store that sells stuff that was donated to it (e.g., Goodwill or Salvation Army in the US). These places often sell books for a dollar or less.

(Because Mormons like to give copies of the Book of Mormon away, there are a lot of them that get taken to thrift shops when non-Mormons get rid of old stuff; because of supply and demand, I think you're less likely to find a copy in a regular used bookstore.)

(Insults to people's holy books rot13ed out of a possibly excessive sense of politeness: Or jnearq; abg sbe abguvat qbrf vg vapyhqr n Obbx bs *Rgure*, nf sebz jung yvggyr V'ir ernq bs vg, vg vf *rira zber obevat* guna gur Wrjvfu naq Puevfgvna fpevcgherf.)