I don't know
specifically about seville oranges (meaning from seville,) but bitter oranges (as they're sometimes called) are often available in that name at caribbean markets: I've seen them sometimes at Marketplace on Oakton, and often at the Rogers Park Fruit Market, but haven't been on the lookout recently. (Sometimes it's called "sour orange," a more literal translation of "naranja agria.")
FWIW I did just see the bottled Goya bitter orange juice at a couple days ago at Marketplace, next to the Mojo Criollo that is a staple in our pantry (thanks yet again JeffB, I love this stuff.) While I'm guessing it isn't the same as fresh, I've used it with success. For reference, my homemade mojo recipe suggests subbing equal parts lime and orange juice for bitter orange if it's unavailable.
Marketplace on Oakton
4817 Oakton St
Skokie, IL 60077
(847) 677-9330
Rogers Park Fruit Market
7401 N Clark St
Chicago, IL 60626
(773) 262-3663