One of the best deals available is the map collection from an educational supplier. eSchoolMaps has a huge collection of U.S. and global maps that are supplied in several formats.
The .emf version is not only compatible with PowerPoint, but already on slides which can be inserted into any presentation! And all can be modified with custom fills, resizing, etc. But best of all, the entire collection is just $40! Look at it here.
Here is a US map I modified for a recent project:
– Troy @ TLC