Watched this thread but it hasn't seemed to advance much. You're going to have to reconcile yourself to some things.
JETX is right. There are many different ways to trace, some free, some relatively low cost and some relatively expensive but, if you've never done it before, you may not know where to start or how best to use the information that you have available to you.
You say they were tenants. Even if only briefly, you must know something about them. Did you get a credit app., job information, banking information from rent payments, "emergency contact" numbers, references from prior landlords. The list could go on and be long, but if you want it free or almost free, you're going to have to educate yourself and work. You probably have useful information and don't know how to use it.
And understand a couple of other things. P.I.s can seem expensive but some will charge you a flat rate to provide service and a higher rate only if they're successful. Some P.I.s, who are retired police or F.B.I.,can be successful because they can draw on resources that you can't get to and never will have (part of an "old boys" network) and it comes at a price.
But mostly, P.I.s and skip tracers are using the same tools that are available to you, like public record databases, and those are available online. $25-30 is not free, but it can get you a lot of data that you'd otherwise first have to take the time to learn to gather.
But, unless they simply left a forwarding address, public records can take some time to develop. You may get more information in 3 months than in the first 3 days. New information is established, collected and collated. New telephone numbers get set up. New lawsuits get filed and served at new addresses. Maybe property is purchased, but there's an escrow first. Financing statements get filed for new loans, but the data takes a while.
Your choice to do it yourself or pay for it.