I assume you're mostly asking about portable printers because you'd like it to be small? With most portable printers I've seen they only have two advantages: they're small and they have batteries. You don't need the second part of that, so size must be what you're going after.

I haven't seen many well-reviewed portable printers out there. I have the HP H470 because at the time I got it (about three years ago) it was the best reviewed out there. It works well, but I honestly haven't used it in a couple years.

The problem is that you're going to see a huge markup for portable printers because of the battery. If you don't need the battery, you should look for a small regular printer. Maybe something like this? It's only $77, gets fantastic reviews, and looks like it would be pretty darn small.

...crap, looks like it needs 120v...