gen.lib.rus.ec and bookfi.org both work for me, some time ago I investigated the origin of those collections and apparently they began as a set of 30+ dvds that have been distributed presumably primarily in russia for quite some time.

Especially in math and physics they are every poor graduate students dream. Even large libraries have only one copy of some of those books stuffed away in some underground vault.

First result on uncensored search engines.

Some libgen sites may be blocking IP addresses from certain countries. If so, try another mirror, or use a proxy.