I haven't tried this, but the following blog post describes a recipe for a homemade Mountain Dew / Sun Drop clone:
The Homemade Soda Expert
Use this as a starting point for experimentation and don't filter out the pulp :-)
Your other option is to call Excel Bottling over in Breeze, IL (618-526-7159) and ask them where you can by Ski Pop in St. Charles.
If you try the recipe, please let us know how this works for you. I was born in Wellston, Ohio -- home of the Gem Bottling Company. So, I grew up drinking lots of Double Cola and Ski Pop.