Once you file suit, it's the process server's job to get ahold of you. They have less restrictions. And you can tack on court costs to the suit you bring -- and then use the grievance you win to garnish their wages.
It sounds easier than it looks, but that's how you get your money as a debt collector. The ones you can't find are probably just as like to be no shows in court. Once you have a greviance, it's easy to garnish. All that involves is contacting their employer.
You can't "tack on court costs" so easily. In most civil cases in the USA, each party is responsible for his own attorney's fees and costs. As I sadly know from personal experience trying to recover money from an insurance company.
In most cases, any amount under $10,000 is not going to be worth going to court over. You will pay lawyers that much just to get the case to trial.
Most debt collections agencies have a lawyer or lawyers on retainer for going to court. They can, and will, go to court for judgments as small as $1,000. The hearing is often the local magistrate, and unless there is something atypical complicating your case, the ruling will happen in 30 minutes or less. Having proof that you own a debt is a binary thing -- either you have it or you don't.