If I am about to generate one-time pad, where should I generate my pads from? If I want to generate a few one-time pads, is it OK to just read required number of bytes from /dev/urandom without weakening information-theoretical security?
/dev/urandom is only computationally secure, so you won't get information-theoretical security for your OTP if you draw it from /dev/urandom. If you're happy with computational security, you might as well use a stream cipher instead of a OTP. Stream ciphers are much easier to use securely than OTPs.

On Linux /dev/random aims for information-theoretical security, but in exchange you typically get pretty bad performance. How well it achieves that goal is practice is unclear, since it relies on entropy estimators for its entropy sources, and that's error prone. (I think on BSD /dev/random is equivalent to /dev/urandom because they don't care about information-theoretical security)