Do you need a feed at the projector or second laptop?
Either way, it should be pretty easy to get a mono feed from the FOH mixer. Most venues have audio patch so it's unlikely you'd be more than 100' from one so carry a 100' balanced (TRS) 1/4" jack cable with you + a 1/4" to 1/8" adapter (assuming you've got a mic input on your laptop) and you should be ready for most situations but most venues should have that in their inventory.
You can always get a cheap audio to USB adapter:
http://amzn.com/B001MSS6CS and just carry adapters from XLR or 1/4TRS to 1/8".