Try ebay or other forums, i.e Corral.net or SVTperformance. I think both of those parts make a huge difference in the look of the car. Be prepared to spend at least 3-400 for mirrors, and worst case scenario for the side skirts, you can get them from Ford for like 6-700 painted, so I'm told.
Keep checking on modularfords.com in the Classifieds under Exterior Parts and Parting Out a Car. There are a few mirrors on there now. Usually used cobra mirrors and side skirts go for $350. If you need to get them painted it will be over $1k.