Of course you can buy on-line at many fine internet book stores, but why not support your local bookstore and buy it there?
Update 2011: every single gay or lesbian or feminist bookstore I listed here has closed. The world has changed.
Amazon, the company that has done so much to close bookstores, yet of course has done so much to make buying books quick and easy, is now the destination of choice: Amazon.com.
You can also buy from Barnes and Noble online store: Barnes & Noble
In Canada, Chapters/Indigo will let you buy Canadian: Chapters/Indigo
I do not have any financial arrangement with any online bookshop, so buying from these links doesn't benefit me. Choose to buy as it suits you.
In the U.K., go to the following page.
In Spain (both Castilian and Catalan), go here.