I love the lore of the 40k universe, out of my group I'm the most knowledgable about it. But I don't have it in me to make my own adventures.

Is there a site or something where other people have posted their adventures to share/sell or something?

I'd just like to be able to scour a site until I find a mission that sounds fun to try and print it off.

Check out Dark Reign. I'm not using premades myself, so I can't comment on their quality directly, but I have a very high opinion of the site in general, so you should easily find something to suit your taste.

Morangias said:

DarkReign40K ist not bad place, but they never sorted the "okay, this my campaigne backdrop/idea" from the "this in actually written as an mission/scenario" so you will have to do some digging. I suggest the following titles (you can find here)

http://darkreign40k.com/drjoomla/index.php/threats/scenarios

Bunker DS506 (a little short, but usable)
The Arastus Affair (not my style, but rather complete)
Grey Matter

In adition I would suggest the following scenario contest modules / freebees

http://www.fantasyflightgames.com/edge_minisite_sec.asp?eidm=50&esem=4
Idyl Heresies
The Brutal Lament

http://www.fantasyflightgames.com/edge_minisite_sec.asp?eidm=78&esem=4
DarkFrontier could be easily turned into a DH mission. The ship is a transport and the pc are send to bring an artefact from A to B. The artefact.
The Glass Key - IF you could out the begining.

Yep, Dark Reign is the place for fan-made material.

The main site's a little hard to get about but as i understand it, a revamp is in the pipeline.

Even so, there should be a lot of adventures and ideas over their to keep you going.