try LOFT or Tokyu Hands, they got a great selection. I think it is a great time to buy postcards now because they got Christmas and year of rabbit cards!
LOFT and Tokyu Hands should have all kinds of postcards, but If you want more touristy ones, I think the vendors next to Senso-Ji do sell views or illustrations of Japan postcards (but not sure if they are still open due to COVID)
Also, Tokyo Disneyland/sea sells beautiful postcards. I also love collecting postcards from Snoopy Town, there are a few shops around Tokyo. Snoopy town postcards are all location specific.
I remember the Sanrio shop on Tokyo Sky Tree got cute Sanrio postcards, and there is a animate (?) store there that carries anime related postcards. The gift shop on the ground floor got postcards related to Tokyo Sky Tree. I remember buying Stitch postcards, he was touring Tokyo Sky Tree on the cards. Also the underground shopping street under Tokyo Station got postcards in most anime/character shop, the postcards are related to the theme of the shop though, so not touristy.
If you are looking for shaped cards, Gotochi cards are sold at postoffices. I personally like their shaped mailbox postcards better. They also sell standard size ones. Central post offices carry better selection of postcards and stamps. I feel it is the easiest to buy stamps from Tokyo Central post office. They got a flipping area with sample stamps, and you write down the stamp ID and the amount you want, bring it to the counter. Not much misunderstanding can happen LOL
Really jealous you are in Tokyo! I really want to go to Japan again (for the 14th time :P), but tickets are really expensive and I’m not sure if I want a 3rd shot or get tested before I go.
I grew up in Texas, but not sure what part you live in. I lived in the DFW area, they do sell postcards at the University bookstore, souvenir shops, some Walmarts, museums, and tourist traps LOL I heard there are LP postcards sold at most Cracker Barrel, if you got one near your place you should go check it out. Lantern Press postcards are pretty popular on postcrossing.