Let's see, it will be wherever you've stored your files in the computer. I've got shortcuts on my desktop, so unless you've done that it won't be the same. However, wherever you have your files stored, I believe it was under some folder called Systems (not the one in the RTP) and then Extras.
If not, somebody uploaded them to Mediafire at:http://www.mediafire.com/?mj3hqdzzjzd
There's also a few very basic maps provided in Degica's tutorial at RMWeb at http://www.rpgmakerweb.com/support/tutorial/rpg-maker-vx
Scroll to the bottom for the bonus lecture on mapping tips, open it, scroll down to where the download is. These maps here aren't as good as VX's sample maps, which are just about average to begin with, but they can be stepping stones for your own, and the tutorial provides useful tips on making them come alive. Also, as an FYI, VXA has a ton of sample maps to (about 2 and 1/2 the size of what's included in VX), and these are built into the map editor where you can simply have them uploaded into your project. Most of them also look very well done.
Edit: They just got one of each, lava, ice, rock. They don't do a lot of outside fields (desert, mountain paths, etc) which I thought they could have done.
Finally, one last thought, First Seed Materials also has sample maps for their Mack tilesets. From my understanding of the terms (using Google translate) these maps can also be used in your games, but I always encourage people to use them as a sort of base or to help get creative, and then make your own.
First Seed Material's Sample Maps using Mack tiles:http://translate.google.com/translate?sl=a...%2F&act=url