All of the .com and .net NNN and NNNN are taken. You might be able to find some NNNN in .org, .info, and .us though don't expect any good numbers to be available.
i had a thread where there is a list of all combinations of 3 characters/letters.com in 8 files, but i did not run any whois on them as i do not like to do bulk whois.
anyway thread is at http://namepros.com/showthread.php?t=130676