Here are a few links to companies or photographers that I have found useful
I have used his site for quite a while. I didn't really understand astrophotography until I took his workshop. HIs tutorials on luminosity masks are also highly recommended
Canon Explorer of light and all round good guy. I have take four of his workshops and come away with something new every time.
I use this company for most of my overseas travel. The guides are always great. The group is always small (max 16). You get a great understanding of the country but also the people. Best of all no single supplement.
A local lens rental company who also ship nationally. Very easy to work with