Do you want a onesie? Or a "one piece"/footed sleeper/sleeper? Because their is a large difference. Onesies, which I learned are a trademark by Gerber are the t-shirts with the snap crotches. While a one piece or sleeper is the one piece jammy type thing with a zipper and sometimes feet. It always bothers me greatly when people call sleepers Onesies. There is a very huge and clear difference, and is it kinda annoying and stupid people now think one is the other.
But check out all in one (they call them onesies too, grr, damn you UK, get it right) company. They are pretty expensive, over $100 or more for a onesie, but it's custom made and you can decide everything about it, from zipper style, to pockets, to patterns, and even hoods with ears and tails.