Hey @BradGreen,
There are some portable printers available that will work with iOS devices. You can view them here, you'll want the Star Micronics SM series bluetooth printers.
I don't have a personal recommendation of those models to make, if you're keen on one of those I'd see what's currently available, I imagine they operate in similar ways. Unfortunately there aren't any compatible with Android, though you could look at Square Terminal, which has a built in receipt printer (it will need internet to run, you can use mobile wifi or a personal hotspot on the go).
