> You can get provider-aggretable (PA) IP addresses from your ISP, but they can only be used with that ISP.

Sorry, this is incorrect -- I'm a network engineer and I've been doing exactly this for years. I advertise "Level3 IPs" to other ISPs/peers, I advertise "non-Level3 IPs" to Level3, hell, I even "re-advertise" some of my customers' PI addresses (that they advertise to me) up to Level3.

Actually, I can advertise any prefixes I want via Level3 -- even yours (assuming you had your own).

n.b.: Now, some ISPs may require an LOA (or similar) before they'll accept certain prefixes from you but that's a (easily solved) procedural issue -- not a technical issue.