check the canal web site: http://www.pancanal.com/
I believe that transit for private yachts is on Tuesdays and Thursdays. You can find line handlers there - many are yachties waiting to transit themselves and want the experience of locking through on someone else's boat. We picked up two backpackers who got transit to the other side of the peninsula and had an interesting experience as well. Used to be US military personnel on their days off would also work as line handlers, but I'm not sure if there are many of them around anymore.
Before you can transit the canal your boat needs to be measured by the canal authority. This measurement has nothing to do with measurements or dimensions of your boat, so nothing you can provide if it's not Canal Authority admeasurement, will be accepted. But once it's done, they assign and official number to your boat, and this admeasurement will be accepted the next time your boat transits the canal.
Since you need to be measured at the canal zone, you cannot schedule your transit in advance of arriving in the CZ. I have no idea what the costs are nowadays. You do not need to pay ridiculous prices to an "agent", you can do all the running around yourself - it doesn't take long, and everything is nearby. Agents are for freighters and cruise ships, and price their time accordingly.
It's a beautiful trip. I was surprised at how lovely the area is! (except for Colon - horrible place). Read some accounts of people who have gone through. it is usually fairly easy, but sometimes things get away from people. You need lots of line - 4 lengths X 200' (I believe). Our friends bought cheap polypropylene line right there in the Canal zone. And you need lots of fenders or borrow old tires (but make sure they're covered to protect your freeboard.