I read this thread the other day, and it got me wanting to try out a onesie, but with me not having a job, I just can't jump and buy one that's premade. So I went looking for T-shirts and I found this T-shirt and thought it would be nearly perfect for a onesie. But even that T-shirt is quite expensive for cutting up.
Here's one that has multiple layers and a zipper on each side, might be able to retrofit it using those layers, however, again, it's even expensive.
It's hard to find a under T-shirt for me, I'm skinny but tall, so I'm always scared of buying/ordering T-shirts for them not being long enough. Even regular shirts, I like to have a bit longer. I usually wear Large but some shirts I have had to go to XL for length, but the chest ends up being so much larger than what I need.