I made it at TechShop. http://www.techshop.ws
Several years ago, I saw resin adirandack chairs with integrated (molded) cup holders. When I went back to the store to buy them, they had stopped carrying them, and have not been able to find them since. You can purchase add-on cup holders, but at $25 + shipping, that's more than you'll pay for the entire chair!
Here's a super easy way of adding cup holders to your chairs, and may actually be free!
Step 1: Mark your location
Trace the bottom of the cup with a marker or pencil.