Best Skip Trace for Cold Calling

[Matthew John]

Hey guys, 

I've been doing a fair bit of Driving for Dollars in my target market & I've accumulated a list of around 50 properties. 

I tried mailing about 15 of them directly with a hand written letter, but I didn't receive any response. I figured cold calling the owners of these vacant houses would be best. 

I already looked up all the owner's information on County Records, but I'm unable to find a reliable website to consistently generate an active phone number for them. I've tried Whitepages and Spokeo, but I've had little luck. 

What do you guys recommend as the best skip trace service?