The two options I prefer are to either buy an unlocked 4G router on amazon or just buy an unlocked phone and use the mobile sharing option. I've done both and they've both worked well. I prefer the phone method since I can then use it as a phone as well.

You could also do the same with an unlocked tablet.

Sim cards can just be bought ahead of time from mrsimcard.com - little more expensive then doing it there, but saves you time.