T.S.
I remember onesies stopping around that size when my son (now 4) was little. Although it seems like you'll never survive without them, by the time your son is wearing 3T and 4T, you won't want to be snapping things under his crotch (he'll probably want to dress himself and he'll be trying to use the potty). NOW with summer around the corner, with a fully mobile toddler, you'll probably find the regular undershirt styles (available in the baby isle with the training pants, onesies and socks) are MUCH more convenient for running around in a diaper and t-shirt outside, and making diaper changes quick.
Sorry i can't help with FINDING onesies, but hopefully this helps you feel comfortable trying the underthings that ARE available in your boy's size.
T..