Your best bet is to use an API like domaintools.com (whois.sc) or write your own whois crawler, which is a lot of processing.. every single domain name registered, and you crawl 2-63 characters long, i have character lists for 2s,3s,4s,5s all the way up to 63 characters long (alpha num [0-9a-z]) but not for sale or going to give them to you. It takes a lot of processing to go this route to be 100% self-sufficient, and actually keeping track of every little detail.. it would take a cluster of servers or even a couple to handle this load.
Also, i know for a fact that onsamehost is wrong and so is myipneighbors for the sites i checked, including mine.
http://whois.sc/cselzer.com compared cselzer.com to onsamehost and myipneighbors you will notice there is a vast difference. (you will see a thing that says sites hosted on this server)
It seems like they don't do a 100% thorough whois crawl, so their data is poor, while domaintools does. (domain tools = whois.sc's original domain, just used to typing it like whois.sc)
EDIT: Was doing other things while writing this post, johnch beat me to domaintools.