Try using http://justcause.wikia.com/wiki/Main_PageFound this link on the Just Cause 2 forums. I do not take credit for this, am just lifting the link.
Pictured and Referenced collection of vehicles in the game - IT IS NOT COMPLETE YET, but most vehicles have pictures of them, and most have their spawn co-ordinates.
http://www.justcause2.moonfruit.com/Credit and Thanks goes to helloeveryoneee on the Uk gamespot forums.
PS - I just finished my achievement; I had it easy with the limited edition set (I think you get 3-4 bonus vehicles from the agency) and I'd like to advise you to type in "[Vehicle Name] Just cause 2" into a search engine, and browse through the results, most of them should be forum entries on various sites giving advice where and how to collect them.
Good luck!
EDIT: Found a replacement link for everybody still needing this achievement. Using the wiki page is also good. Best of luck!