gerber and child of mine sell undershirts (pull on, but I've only found short sleeves) and I dye them (but I love a white shirt) and that's all I've found (at walmart) and the only other l/s shirt I have came with an outfit from old navy. Also 2 piece pajamas come in 3-6m at old navy and I use the tops for hanging around with different pants. Hope that helps.. Too hard to find.. maybe a good reason for that.
If I did the coverless thing I usually just left her onesie unsnapped, this was when she was too little to get tangled in it though