The terminal is called Albrook Bus Terminal. We went there from the Panama Canal by Bus as the Taxis were an absolute ripoff. We wanted to go initially to David, but then we decided to get the bus till the boarder with Costa Rica (La Frontera). 

If you don't want to go with the Tica Bus, then your only option is to get your ticket the same day as the departure. For us was a bit inconvenient as we were just coming back from Islas San Blas the same day, but the guys from the transportation company told us they can have several buses per hour depending on the demand. We took the 11 PM bus and they were 3 from the same company leaving at that hour.  

Cost of the bus was 20.25$ per person with arrival at 7AM. Taking time change into account. 
For the bus trip, take something warm with you as it is cold inside, the seats are decently comfortable. 

woke up in the morning and stopped for breakfast at a restaurant that has been recommend by a friend as having the best traditional breakfast in town. The place is called El Trapiche and below you can see a picture of the breakfast I had.

from here, we ordered an Uber to bring us to the Panama Canal. I don't remember how much it was, but definitely had a decent price, I highly recommend using Uber while in Panama City.

Bring your student card (if you have one) for the visiting the Canal (Miraflores Visitors Center) as they have a discount. The price was 10$.