Clay,
Museums are an excellent source for old maps. Granted, they will be in a display, but with a donation to the museum you may be able to get a copy. Often times, the maps you seek are still "in storage," which means that you have to specifically ask for them. Some museums have a research library that you may, or may not, get access to. You'll have to check it out.
If the old files and maps haven't been put in/taken to the new facilities, then ask where they are being kept, they just don't throw this stuff away. On the other hand, with the information about these maps that has been revealed in the last few years, it may be impossible to even get questions answered as to the where-a-bouts of these articles.
The Internet used to be a great place to find old treasure maps but, once again, with the info that has been revealed in the last few years, sites that contain any maps of "value" are on the decline, or, the maps that lead to treasure rooms have been removed from public access.
Books are another excellent source for these orignal maps. Many towns have printed a book that was released during the towns centennial. These books tell the story of the towns founding, its expansion, etc., and many times copies of the original plat maps will be found in these books.
The list goes on and on. You have to not only use your imagination as to where to look for these maps, but you also have to have a good eye for these types of treasure maps, that way you can spot them at any time and in any place.
Also, you must understand letter-style treasure maps. These treasure maps are just as effective as plat maps, but their much harder to recognize, unless you can spot the tell-tale signs that say to you "this is a treasure map."
There are a few sites on the Internet that sell old documents. These used to be a "clearing house" for free treasure maps (all you had to do was copy them), especially the letter-style treasure maps. Lately, this has changed.
The Library of Congress has millions of maps that lead to treasure, as well it should. Many county maps can be found here.
Again, the list goes on and on.
