Temple Assembly Site
Video needed.
Discovered by Frozen Mr. Toastbrot.
I also posted this in the forums, so I'll just copy what I wrote there.
I found out, that you can get the Missile Expansion in Temple Assembly Site before the main Missile Launcher (right after loosing your items). The trick is to jump onto the pillar small that is the most left when you enter the room from Collapsed Tunnel. To get onto the piller with a single jump is quite tricky, and I still don't know how to make it onto the pillar for sure (that's why I won't post it in the wiki yet), though I could do the jump a few times. All I know for sure right now is, that you have to use the small slope on the ledge in front of the pillar (I hope you'll understand what I wrote) get a ghetto effect. It also seems to help if you start the jump a bit right of the crack in the slope (the crack is next to the wall).