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.
Step 3: The container
You can also remove the label. (We only left it on for the photos.)
Drop the container(s) in your chair and enjoy. And because the holders are not fastened in any way, they do not interfere with stacking.
UPDATE June 4, 2012:
UPDATE June 9, 2012:
I have been designing products in the automotive, aerospace, medical, defense and consumer products industries for over 30 years. I am an "Inventor for Hire", so if there is something you need designed or built, just send me a message, or stop by http://www.lime3D.net, and maybe we can do business!