Japantown has some vegetarian restaurants, and since they get a lot of tourists they're used to specific food requests. Just ask before you sit down. There's also a Japanese supermarket there and you can check the ingredients list on the packaged food in the refrigerated section.

You'll find a million places to get them if you search yelp or google maps but the ones most like the article are likely found at asian grocery stores.

Woori or Nijiya both have a good variety of them and should have some vegetarian ones like pickled plum.

Across the bay at Berkeley Bowl they make some an surprisingly good ones too.

Nijiya Market in Japantown has at least plain seaweed Onigiri in far right of the fridge near the entrance for $2.79. Unfortunately it was the same price as ones with filling.

I'd love to know the same for Barcelona :)

I bought some Onigiri at the Manga Salon (a big annual manga convention) but I have no idea where they are available year-round.

There is now a US chain for onigiri: Onigilly. About a third of the menu is vegan and gluten-free.