There are a lot of companies that provide domain whois but I've heard of a lot of people who had bad experiences where the domain was bought soon after the whois search and the price was increased dramatically.
Where can I gain access to a domain whois where I don't have to worry about that happening?
Apparently, the official name for this practice is called Domain Front Running and some sites go as far as to create explicit policies stating that they don't do it.
This is where a domain registrar or an intermediary (like a domain lookup site) mines the searches for possibly attractive domains and then either sells the data to a third-party, or goes ahead and registers the name themselves ahead of you. In one case a registrar took advantage of what's known as the "grace period" and registered every single domain users looked up through them and held on to them for 5 days before releasing them back into the pool at no cost to themselves.
And apparently, after ICANN was notified of the practice, they wrote it off as a coincidence of random 'domain tasting'.
Source: See for yourself