I got mine off of eBay but then I found much better ones from Adult Cloth Diaper in the US and BabyKins in Canada. Mind you that they generally have solid colors with no prints but then this gives you the ability to have whatever you want printed on them.
I have had other shirts printed at a variety of local shirt printers so I would imagine a onesie will not be too much of a problem.
There are lots of other places that sale them but I am not aware of any that allow you to do a custom print.
Well get the oneseies from babykins or ebay then find some one like in the mall there is a guy that will airbrush anything.
So there you are.
There are airbrush artist in the mall were i live.
It is a tee shirt shop they air brush tee shirts you bring in a photo and they airbrush what is in the photo.
So you should be able to find some one to do it for you.
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Just find out someone who can print shirts and all these stuff, in my country is very cheap (And can find it almost everywhere).
Btw, I don't even think is a really good idea go to a IRL shop and ask for it, but anyway, isn't seller business why you want that print.