Google is your best friend in these instances. Type in the keyword of the domain you would like to find end users for and take the top 20 or so. Find their contact info or run a whois search on the domain itself. I have found that emailing the contact info with a cc to the whois email works best.
Other than that not exactly sure of a better way to find targeted end-users' emails.
First of all you need domains that have a clear use. It is not easy to sell a "brandable" to an end user. Generally end users are looking for something like keywords or acronyms that have a clear use and value to their business.
1.) Find similar developed sites. In general it works best to find a worse version of the domain you are trying to sell.
2.) If it is a keyword domain, find the PPC sponsors on Google.
3.) Look at the top results for the term in Google and Yahoo. See if the match is relevant and it could be a potential target.
use the keyword from your domain and search it using google.com usually if they own a domain they have contact form use that first if there's no available form then go with the email in the domain whois because they need to clean that email and check it always.