First, gain access to Kanto. Then, head to the Sea Cottage (i.e. Bill's house) north of Cerulean City. You'll find his grandfather inside. The grandfather wants to see a variety of Pokemon (which he describes to you, rather than actually naming), and he'll give you evolution stones in exchange. Just keep showing him Pokemon, and eventually, he'll ask to see a popular yellow Pokemon with red cheeks. Show him a Pikachu, and he'll give you a Thunder Stone.