Robert Royce - http://www.rfroyce.com
Conical primary mirrors, Dall-Kirkham and Newtonian OTA's

Protostar - http://www.fpi-protostar.com/
Blacklite® telescope tubes, spiders, secondary mirrors, flocking materials.

Hastings Pipe Company - http://hastingsirrigation.com/
Aluminum telescope tubes in various sizes and wall-thicknesses. Rolled ends available.

White Cap Construction Supply - http://www.whitecap.com/
Sonotubes in various sizes with larger ID's frequently available.

IP4AP - http://www.ip4ap.com/
Image Processing Lessons & Tutorials - Education for Astrophotographers of All Skill Levels

HyperCams & Mods - http://hyperdslr-mods.blogspot.com/
DSLR Modifications - TEC Cooling - Filter Replacement for Enhanced Spectral Sensitivity

Astro Laugh - http://www.astrolaugh.com/
Humorous Astronomy Apparel

Prepping a Telescope Tube for Mirror Cell Installation
How to Mark Evenly-Spaced Mirror Cell Mounting Holes - PDF