"onahole" is オナホール, but you won't find a signboard with that written on it directly on the street ^^ Shops selling sextoys and adult stuff must be registered as such, and are often in the backside of "wellness and relaxation" shops or on Nth floors of really random-looking buildings. If you don't know the address beforehand, you'll pass by it for sure.

The few "brick and mortar" shops I heard of are in Tokyo, including Love Merci. You can try randomly googling アダルト (adult) shops in 札幌市 Sapporo and take a look if they sell what you need, but Amazon Japan will remain much much cheaper.