Hi Lisa,
What a lovely idea - they really do make a great keepsake.
There are several places at Walt Disney World where you can have silhouettes done.
At the Magic Kingdom, there is a large cart situated about half way down Main Street U.S.A on the right, just into a small alleyway and you can also find another silhouette cart in Liberty Square.
At Epcot, there are silhouette artists located near the France pavilion in the World Showcase and there is also a silhouette stand at Downtown Disney Marketplace .
Although we haven't had any done for the past couple of years, from memory, the actual silhouettes themselves were approximately $10.00 per person (please do bear in mind that prices can change) and each location does have a selection of reasonably priced oval frames for sale as well.
Wishing you a magical trip filled with memories which will last a lifetime!