If you don't want physical books: https://www.marxists.org/index-mobiles.htm

And if you do want physical books, they also sell them. I don't actually know anything about MIA as an org, but their site is great for research and feeling like it's 1999 again

most college libraries have a great collection and sometimes offer a free card to the general public

bookshop.org for new books. abebooks is probably best for used books, but even they are owned by Amazon. Sometimes you can find books on there and then go to the listed bookstore's own site, but more often than not they'll just be using the abebooks engine there, too.

If you want to support the author, there is always the option to pirate the book, and see if he or she has pages to receive donations (paypal, bitcoin, patreon, etc) or you can just email then money.

I recently bought a cool looking 1920's copy of State and Revolution from eBay for $15. I've had good luck buying used leftist books from there, especially the ones that were printed in large numbers.

To add to the ebook recommendations, Anna's archive has the biggest pirate collection of ebooks online.

ebook-hunter.org

thriftbooks is really good as well

Death to America

For Spanish-speakers, I think that the biggest editor of communist literature in the Spanish language is the Fundación Federico Engels