Greetings. I searched a bit for Space Quest on Google but I couldn't find any place to order it I fear. I did however find a complete remake made for Windows.
I won't be posting it here because it could be warez. I'm unable to verify it, sorry.
Another good place to ask is probably the official sierra community forum.
http://forums.vgames.com/forum.jspa?forumID=111
Try asking people here, perhaps Sierra has some kind of classic game pack you can buy.
Kind Regards
Nick