Hello SD
Is the original artist someone we know? A member here, perhaps?
I ask, because it would be easier to work out how the map was created (eg if it was Photoshop, GIMP, or some other software favoured by the original artist). In addition to that, if the artist by some slim chance just happens to have created a tutorial on this or a similar style, it would be easier for us to find it for you
EDIT: There are lots of parchment textures available in the Mapping Resources forum. If you don't want to go hunting them down thread by thread, however, I have a very small collection linked in my signature below you could choose from.
I created mine in GIMP after watching
this YouTube tutorial.