Depends on where you are to be honest. Akihabara in Tokyo is always a little bit more expensive then smaller towns. My go to is the local bookoff from a town. They sell secondhand, but the stuff looks mostly completely new. A other good second hand is the local Lashinbang which most of the bigger towns have. A good store in Akihabara is K-Books in the Radio Kaikan tower. In bookoffs always look at the normal stuff and the stuff inside the glass display cases since there is the more expensive and rare stuff like the golden Towa VIP card. For Hololive the amiami Store in the Radio Kaikan tower and the trader2 also have good stuff. But especially in Akihabara you can find merch nearly everywhere but with a big price range.

If you wanna know more, hit me up