These are some sites that currently (or will shortly) have Space: 1999 merchandise available.

The Official Gerry Anderson Appreciation Society

Fanderson is the ultimate site for all things Gerry Anderson.
They have released the scores to both Seasons of Space: 1999 on CD, and a documentary on the making of the show.
Click on the logo above to visit their site.

Releasing an all new series of Space: 1999 books
"Written by professional authors, individually chosen and invited to contribute to the series, these novels will build upon the legacy created by the 48 episodes produced during its initial run and appeal, not just to the generation of fans who have kept SPACE: 1999 close to their hearts, but also to readers of first class science fiction."
Click on the logo above for more information, and a link to their site.

"The Home of Collectibles from British Classic TV and Films".
Click on the logo above to visit their site.

The North American Headquarters
For Gerry Anderson Collectibles
and cool toys from around the world!

Network has released Space: 1999 on BluRay
Click on the logo above to visit their site.

If you know of any other sites that should be included here,
please let me know so that we can support them.