I did a search and found a place called Kraft Music that sells a bundle that includes the Guitalele and an extra set of strings. You just use regular guitar strings, but tune up to A and not E. Here is a paste of what it says about the strings.
Extra Strings are a necessity when playing your guitalele. Depending on how much you play, you will want to change strings every 1-2 months. Our GUITAR ESSENTIALS BUNDLE has you covered with your first replacement set. This D'Addario EJ45 Pro-Arte set features a regular gauge that is easy to play, while retaining a full, bright tone. (sells separately for $11.95)