If you went looking for a 50cm beach ball on Amazon, you might find that the 'Frequently Bought Together' field is suggesting something odd -- a bottle of glue, a pack of high powered magnets or even an Arduino board. It's not Amazon's algorithms messing up -- these are the ingredients that are needed to build a homebrew BB-8.
While this instance was pointed out by the BB-8 Builder's Club on Facebook (if you ask them nicely they might even help you build your own roly-poly astromech), the process comes from an Instructable made by a 17 year old student named Angelo. While people are making replica BB-8 droids with everything from 3D printed panels to pre-made plastic hemispheres, Angelo's approach is a lot more accessible to the average DIYer.
The first pioneering BB-8 builders watched the available The Force Awakens footage over and over to figure out the scale for a properly life-sized droid, and figured out that the body was a sphere close to 50cm in diameter. Of course, there are plenty of existing objects that have a 50cm diameter -- not only beach balls but also rubber exercise balls and world globes. The people who want their own BB-8 on a budget are making the best possible use of these odd bits and bobs.
Angelo's process uses only household items (along with your Arduino board) to build a life-sized phone-controlled BB-8, with the materials cost potentially coming in at under $100 (depending on where you live, of course). If Amazon's 'Frequently Bought' is anything to go by, then there could be a bunch of these paper mache BB-8s rolling around right now. Move over, Sphero!