A quick tip regarding searching for Russian books online: when you’re searching, try the transliterated spelling of the title / author as well as the original Cyrillic version. I was recently trying to find a Russian history book for learners of Russian, Страницы истории by С.Н. Сыров. I had found a few copies from sellers in Russia, but payment methods were a problem. However, a few more copies popped up when I switched to searching for Stranitsy istorii by Syrov. I guess it’s logical enough that a book seller listing stuff on an English-based website wouldn’t list things in Cyrillic. Abebooks in particular won’t even accept searches in Cyrillic; if you enter Cyrillic into their search box, it comes up as having searched for a bunch of nonsense characters.
Online libraries:
http://www.koob.ru
http://www.lib.ru