Someone handed down some Gerber shirts to us, and I also found some by "Bon Bebe" at Marshall's/TJ Maxx. The side-snap ones work well, too, and I've seen those all over the place. On that note, too, Sam should be big enough to move into our CD's next week...yippee!!!!
we have a big stack of the side snap shirts, plus a bunch of onesies that seem to fit fine over our diaps; though, i think we'll have to size up sooner rather than later. i also had a bunch of the babystyle kimono style onesies that i got on sale a long time ago ... and they fit over quite well. i'm very into stuff that doesn't go over his head these days - he hates when i try to get an overhead onesie on him.
otoh, he's mostly in a diaper & cover only these days anyway.
Seriously, I do cut a lot of my onesies, but I've also found regular tees here and there. If you're googling or froogling or ebaying, try "infant undershirt" or "infant kimono" for that type that wraps across the front. I like those because baby doesn't really like me pulling stuff over his head. As in "noooo, I was already born. Not this again!"
oh! if you're having trouble snapping them and don't mind the diaper/cover hanging out (i mean, if it's a practical matter rather than an aesthetic one), you can get onesie snap extenders, which give you a couple of inches more length. (does that make sense?)