The publication rights of Mein Kampf have entered the public domain fairly recently, before that having been held by the Bavarian government. This older answer by u/Georgy_K_Zhukov goes into some detail about it.

That said, by all accounts (I haven't read it), Mein Kampf is a fairly atrociously written book, and not of a ton of use in studying Nazi ideology.

u/Georgy_K_Zhukov also discussed his opinion of English translations in this thread. I noticed a couple of new copies (of the Manheim translation mentioned in the thread) in my local discount bookstore two days ago although I didn't check the edition, so they should be readily available.

https://www.reddit.com/r/AskHistorians/comments/4n4m0g/good_whole_text_mein_kampf_english_version/