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.
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