There have been a couple calls for a buyfag thread, so here we are. I've just slapped together a few book-related links here, so please volunteer any additional resources or information you may have about yuri purchasing. Not entirely sure how we're going to keep this alive after any initial questions are answered, perhaps we could post collections or recent orders?

General guide on buying yuri manga
http://divulge-scans.tumblr.com/buymanga

/a/'s buyfag guide (some relevant info, incl: customs, shipping, book and doujinshi retailers, alternate honto signup guide, etc)
http://buyfag.moe

How to signup for honto:
https://kaizora.wordpress.com/2012/05/19/how-to-signup-for-honto/

How to purchase and convert a JP kindle book
http://pastebin.com/j5n5zSuN