I'd like to use an N number that is listed as available. But when I request it, it shows as being deregistered. How can I get this number for my aircraft?
If the number is deregistered, you cannot re-register it unless you have the previous registered owner of that number submit a request to relinquish the number to you.
So you must get in touch with the old owner of that N number, and if he is willing to write a letter relinquishing it, all you should have to do is send his letter in with your request and a check.
If you have any other doubt, this page has information about your issue: About Aircraft Registration Renewal
If you need any other information, do not hesitate to comment and I will try to give you a more detailed answer for your problem