Why let others do something you can do yourself?

Garnishment process is basically these:
1) File garnishment with tenants' company HR department to deduct payment from their pay check. This is very effective as long as you know where they work. Try follow them in the morning to their work place, you will find out which company they work for.
2) File garnishment form with State Treasury Bureau to deduct payment from their tax refund.
3) File garnishment form with IRS to deduct payment from their Fed return.
...

You can do all that, right? why pay him 50% to do that?

Never use collection agency, most of the time they won't get a penny back for you, a total waste of time.