My business is in the D.C. area. Can anyone recommend a good collection agency that will help me to get a "client" to pay up on his outstanding invoices?

I have learned my lesson, and I'll never make the mistake of not trusting my instincts ever again. I have come across a "client" who REFUSES to pay me for the services I provided him. I have tried to collect via numerous telephone calls and going straight to his accounting dept., to no avail. It is costing me time and therefore losing me money. It's time I hand the matter over to a collection agency. Argh.