They are sold on the big auction site.
There's a forum which shall not be named which is geared for a
particular aspect of tennis - that particular aspect isn't really discussed much there - the main activity is to buy and sell pro stocks
There are a few people on tennis boards that sell a fair number of them - you can just ask if you know who they are
Some people know players that get them and sometimes they get a freebie or are offered one or more for sale