Hi Keith , thanks for your message.  I'm happy  you like the billboards!  If you have access to a "wall wart" type USB power adapter, like an iPad or iPhone charger, then the best recommendation would be our USB cable, which connects to the output on that charger.  You can also use a USB battery bank-- we have recommendations here.

Once you have that power input, then the rest depends on how far away from each other the billboards will be.  If they are within 24" of each other, then you could tackle everything with these items:

1. 1-to-4 Breakout Adapter with Standard Connectors (Pack of 2) (one 2-pack)
2. 24" Thick Connecting Cables (Pack of 4) (one 4-pack)
3. 1-to-1 Extension Adapter with Standard Connectors (5 Pack) (one 5-pack)

This would let you run beefy power right up to the billboards and power them directly from one USB power source.  You'd also have one open connector on the 1-to-4 breakout adapter, and could purchase additional connecting cables to attach the second 1-to-4 adapter in that 2-pack, which would then get you started with plenty of connection points to start lighting your city.  To get you started on that, we outline two connection methods for buildings in the instructions for our Light Strip Starter Kit.  When looking at those diagrams, in your mind, replace the battery pack with your USB power source, and hopefully that helps.

FYI we have engineered our two main starter kits (Light Strip and Pico LED) to work with pretty much any single building.  Typically people will order a few of each to get started and to get the hang of how the lights work together, then from that point, people often purchase from our bulk packs to add more lights, wires, and adapters.

I hope this information is helpful-- please let me know if you have any additional questions, and there are also plenty of posts from customers who have lit their modular buildings and LEGO cities using our products.  They can be of great help as well.

Thank you again.