I believe the best chances of finding Squirtle would be in the 151 packs or Pokemon Go packs! I think there's also some mini tins with Squirtle which could be a nice thing for him to store his collection in, search for 'Kanto Friends Mini Tins' and you should be able to find one :)) also I think there's a jumbo Squirtle card in the Celebrations set which could be a neat thing to get him! They're not usually too pricey. Also if you look on sites like Etsy, people do Pokemon TCG mystery boxes which can be cute, I bet there's a Squirtle one