Are you looking for licensed, or just decoration? For my decoration ones, I hit up Amazon.com. They have a bunch on there for $0.03, plus $10 shipping, and since they're all from the same place you can just combine shipping and have like 10 posters for under $15 lol
For official, licensed stuff, Ferrari dealerships or store.ferrari.com/en (the online store is doing free worldwide shipping right now)