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.
Step 2: Cutting the chair
Using a jigsaw, cut out the circle. You will want a relatively loose fit, so cut right on the line. Be careful to not cut outside the line, or your cup holder might fall through!
Test fit the container in the hole. If it is too tight, you can touch up the edges with a file.