Collections
AIDS-Selos - The largest collection of AIDS stamps on the web apart from my own. This site is not in English so I have linked to it via an Alta-Vista translator.
Hennefeld Collection - A collection about gay and lesbian history.
Good places to buy stamps
Ebay - The world's largest auction site has dozens of auctions of stamps about AIDS at any given time. Some great deals are to be had on stamps if you look carefully.
Zillions of Stamps - ZoS searches the catalogs of numerous companies. They tend to have minimum orders which makes buying individual stamps difficult, but it's well worth looking around the site.
World Stamps - A difficult site to find what you are looking for, but they have extensive catalogs of some difficult to find stamps about AIDS.
Delcampe - Outside Ebay, the single-largest auction site of stamps about AIDS. Great place to find related materials such as postcards, telephone cards, and First Day Covers about AIDS.
Bombay Stamps - Another good place to buy AIDS stamps.
Organizations
GLHSC - Gay and Lesbian History on Stamps Club. They maintain a good database for members of stamps about AIDS.