A tall white tshirt makes a good alternative to a onesie. Not only do you get to be that toddler hangin' out in a white T, socks, and diaper, but it's tall enough to get down a ways and cover most but not all of it. A good tall tshirt is also just what you need to prevent coming untucked when in public.
I'd been looking for such a thing for awhile, but by the time I got a tshirt that was tall enough, it was like 4xL and wore like a tent. It darn near fell off my shoulders through the neck opening.
I recently found that Walmart occasionally carries Fruit of the Loom tall tshirts. I got one and it was perfect! For my size, 6'1", ~220lbs, (fairly thin but with a bit of a geek gut) I found the "XLT" size was ideal. My Walmart doesn't reliably stock them, so I ended up placing my order online for in-store pickup. A 3pk of these tshirts was $10.64, with no shipping. They have crew and V cut necks available, and also choice of blue or white.
I got http://www.walmart.com/ip/33665820
Beats a single $35-45 onesie that may or may not fit!
If you're not tall like me, and you get one, you could probably turn it into a onesie by adding a button or three or maybe some snaps. (or even just a diaper pin!) to tie the back and front together at the crotch.