What sort of SMBs are you looking for? There are over 5.68m SMBs in the US and you would be hard pressed to find a public list of each one.

However, if you know a location and/or an industry, you can start with a place like hoovers.com or yellowpages.com.

Hope this helps! Let me know if you have any more questions about list building, it's one of the most important steps in any sales process.