There's a best way and a least expensive way:
Least expensive way:
Make a list of every investor you find in an excel spreadsheet or Outlook Contacts:
* Houses for sale in classified ads that have words like "lease/option" and "renovated" or other signs that it's an investor selling it. Call these guys and get their contact info.
* Contact info on people listing houses in Craigslist.org or Kijiji.com
* People who put up Stop Foreclosure or We Buy Houses Signs
* People you meet at REIA meetings
* People who respond to your For Sale ads and signs
The best way:
* All of the above, plus doing some internet marketing to get people to fill out an opt-in form on your website to be on your buyer's list.
This takes some time to learn and some money to get a site with autoresponders, but is totally worth it in regards to the time it can save you. There's a chapter about this in the book This Market Stinks, where Sam Bell talks about how he does it.