Very few private attorney's do pro bono criminal defense work because the need is not there in the community because of the existence of Public Defender's offices. You may want to call around, or check with the local bar associations. Most private pro bono work is done in other areas where there is no other way for people to get free or low cost representation.
If you qualify, you can be appointed a lawyer. Criminal lawyers generally do pro bono work only on post conviction matters, if at all. Moreover, do you really want a lawyer who is spending his or her time working for free on your case? Is there something special about your case that would make a lawyer want to help you for free? If not, then it is not a good idea. If you do not qualify for a court appointed lawyer (also known as public defender), then you need to hire a qualified lawyer to represent you (and that does not always or even often mean the cheapest lawyer).
If its a federal crime, the federal District Court will assign a lawyer to you. Those lawyers are usually quite competent. However, it will depend in large part on whether the Court believes you really need a court appointed lawyer which is based upon whether you have funds of your own to hire your own counsel.