Go back to Akihabara, again :)
"Animete Akihabara" is the largest anime goods shop, up to 4 and 5F.
But finding such small stuffs at used shops in Akihabara may be enough.
Also if enough time, you should go to Animate in "Ikebukuro"
located 30min far NorthWest by JR local train, 200Y/oneway fr Akihabara.https://goo.gl/maps/G4zJk49wnpQ2
and "Nakano Broadway", located about 30min far west by JR local train, 220Y/oneway fr Akihabara.http://www.nbw.jp/#!/enhttps://goo.gl/maps/HxWCkRefnuL2
And, if would like to try to saying(asking) in JP
"One Pun Man goods wa, dokodesuka?(ワンパンマン グッズは、どこですか?)"
(= Where is the goods of One Punch Man?)
Also would be better showing a image of main character "Saitama", if you have a cellphone or tabletPC.https://goo.gl/zVSYk8
Original title is "ワンパンマン(One Pun Man)", just shorted fr "Punch".
Also if bad pronunciation will be another super big hero "アンパンマン(AnnPanMan)", be carefull :)https://en.wikipedia.org/wiki/Anpanman
"Anpan(あんぱん)" is the famous bread, around 1USD very cheap
and getting it very easy at covenience stores and more.https://img1.esimg.jp/blog/wp-content/uploads/2017/04/IMG_4850.jpg
However, most shop staff understand such easy Q, foreigners asked by EN usual, no worry.