I dont know how to get BC and Golden BC.
Go to the daycare in the Bridge Field. Nearby there are two hikers known as the digging duo. In return for 500W they will dig up rewards for you. The one closest to the daycare will only get a few items, but they will generally be better than what the other hiker I’ll give you. The hiker closest to the daycare is the one that you need tot talk to. It will probably take you several thousand W to get them, but you will eventually find that he will give you bottle caps.
Hope I helped!