You should be able to register an exipred or dropped domain at any ICANN accredited registrar or registration provider that is offering the extension that you desire. You may not be able to register the domain until it has completely gone through the expiration and delete cycle. Just because a domain has expired, it does not mean that the origional owner cannot still renew it while it is in the redemption period. You may want to look into some backordering services as well. If it is a great domain, im sure you wont be the only one looking to snatch it up after it expires and becomes available.