Where can I get something that allows me to breath underwater?

You do quests for those boomers up right of the map/some way east of new vegas. One task is to raise a plane from the bottom of a.. sea..? Anyways, when you do that task you get a water breather thingy.

Raise your reputation with the Boomers to Idolized, so they offer you the quest Volare! . Ask Loyal about how to raise the bomber without drowning, and he tells you to talk to Jack. Depending on your Science skill, you can pass a check with him to get the Rebreather right away; otherwise you'll have to scrounge some parts and bring them to him to fix it.

Even if you plan to massacre all the Boomers at some point, it's a good idea to become Idolized with them first, just to get the Rebreather. It's a quest item, so you can't drop it or lose it. It's weightless, and never needs repair or refilling.

refilling?

I meant to say that you don't have to fill it with air or anything - it gives you infinite time under water.

Is that the only way to get one in the game?

It also adds +3 to your DR (but does nothing for your DT).

Yep, through the quest volare! It's rather easy and fun to. Or you could get it through console commands if your on pc