I remember the first time I went to Akihabara back in 2016, I didn’t know where to got. Like a dog chasing cars. There was so much anime merchandise everywhere. It too me a few more visits to understand where to get what I needed ( or wanted hehe ).
But in case you do not have enough time to explore Akihabara, and just want to get a “general overview” on what anime merchandise is out there, visit the Radio Kaikan building. It is just about 50 meters ( 170 feet ) from the “Electric Town Exit” of the JR Akihabara Station.
Other than brand new items, one thing that I found very interesting in Japan ( not just specific to Akihabara ) are the consignment style selling. This is where you can see shelves and/or boxes with pre-loved goods. The items are still in good quality.
There are other ( and bigger ) stores that offer the same style of selling such as Akiba Cultures Zone, Traders etc. But in the case where you are in a hurry and just want to pass by Akihabara and just see random Anime merchandise, I think this is a good place to go.
Another Kirby in Japan 