One good place is in record collecting magazines such as Record Collector (Europe) or Goldmine (US). There are many advertisements in them. As with anything else, some prices are better than others. Ebay also offers some rare Queen items, but can often be very expensive.
For electronic versions of the above, you can check out the Bittorrents on www.queenzone.com. The Queen Hub is a user group of people sharing Gigabytes of rare Queen items. See the Queen hub section for more information.