RMG probably are very good resistors, but there are a few other makes which are very good too if you find it hard to get RMG. Also consider Vishay, Roederstein MK-2 metalfilms or Caddock MK-132.

The following two links are quite good:

http://www.sasaudiolabs.com/theory10.htm

and

http://www.borbelyaudio.com/upgrades.asp