I second the 2xist rec. I have had some that have lasted quite a while now (since 2003 or 2004), and I wear them frequently, as undershirts, at the gym, and around the house for lounging.
You also might want to check out American Apparel for undershirts and clothes like that. Don't be turned away by their 80s retro aesthetic, the quality is pretty good (their main claim to fame is that they strictly control their production facilities and construct all their clothes in L.A. IIRC). Surprisingly, I have found that their "summer shirt" (very-deep v-neck) makes a great undershirt for a casually work button-down, since you can open one or two shirt buttons and not reveal any undershirt. They also have a huge range of sizes.
Armani Exchange makes some nice slim-cut t-shirts that also might work as undershirts, though you will pay a bit more for them, since they are more intended for club wear.