In this post you can findold map brushes, made by m-sign.nl. They are free to download, and this category might be updated regularly with new brushes. The brushes are available for Photoshop: they are in .abr format.
Old-Map Photoshop Brushes Pack
Download the Package:
The brushes were made out of images that were shot by m-sign.nl. The accompanying Old-Map Textures can be downloaded from Textures.
For users of other graphics programs, there are several resources on converting brushes, e.g. to gimp: