If you don't have an onward boarding pass, check at the transit desk. They will either be able to issue you one there, or they will send someone to the departure hall to get it printed out (expect this to take about 15 minutes).

It will help if you have a printout of your itinerary with your record locator if you need a boarding pass.

Also, in checking other posts, it looks like you may not have to pay the departure tax as a transit passenger-- just keep a copy of the boarding pass from your arriving flight.