Pro bono is when an attorney take on your case at no cost. Generally, there are several ways to obtain a pro bono attorney such as contacting your local bar association and requesting the names of attorney who perform this service in your area. If you are facing criminal charges and do not have the money to retain private counsel, a public defender will be appointed to represent you.
Pro bono means "free volunteer work" by an attorney. It is entirely up to an attorney whether they will work for free on any given case. There is no requirement for attorneys in Washington to perform pro bono work. Attorneys will always consider the facts of a case before deciding whether to work for free on that case.
What kind of case are you talking about?