Here's where I got mine.
Visited States Map by epgSoft
Note, I've found it helpful to keep a file that contains the names of the states
we've visited. I also keep that URL above in the file. It's just a word.doc but
you could use notepad just as well.
Using the site above you have to click all the states you want highlighted and
it's easier if you have a list to go by.
Once you've got your states map at their site you can choose size and then
copy the graphic to your desktop. Then you can come here and insert it
in your signature. I use the smallest one but some use the medium size
graphic in their signature lines.