I currently have Photoshop Lightroom 5. I am attempting to use the stamp option in the sketch filter. I can't find it in Lightroom. Upon investigating, I believe that you must use an external editor such as Photoshop Elements. Is this correct? Do you need to purchase this as well?
These are separate programs which can be purchased individually or in combination, your choice. Many people use Lightroom for organizing and editing, and then they also use Photoshop Elements for additional editing power which goes beyond the feature set of Lightroom. Since Lightroom does not have a stamp option in the sketch filter (it doesn't even have a sketch filter), this would be a case where if you needed to do that you would have to use Photoshop Elements or the full Photoshop.