I believe the only place to buy an extra controller is on the Google website store. I’m sure in the future Walmart and target and other major retailers will have a Stadia controller. Although there might be some reluctance to sell the controller based on the fact that stadia could cut into their gaming sales.
@Dogfish :
I've not seen any retailers selling Stadia merchandise yet. Perhaps that will change in future, but currently everything goes through the Google store AFAIK.
Ordering just a controller can be found here: https://store.google.com/product/stadia_controller