
Been there myself. If you can't find it, you're clearly not looking hard enough. Two helpful hints though:
1) Wear a Bob Marley and/or Phish shirt and walk around any reasonably crowded place in Philly where all the cool college folk hang, and someone is bound to ask you if you're looking to score. (Not going to get any more specific than this, sorry)
2) Wait for Phish or some other hippie-friendly music gathering to come around. Make a crude sign that says, "Need ganja for hemp making project," and just wait for someone to come along. (Seriously, it will work)