I don’t blame you, I would want to know about it to.
Although I have to ask. If there were autism-specific clothing companies who customize clothing for autistic people who request them, would they also make animal onesies for me or other autistic people who want some.
Apparently, no one knows the answer to your question. I think you're going to have to explore your options directly with any such company, if such a company even exists. If there are no such companies, your best bet is to hire a seamstress or tailor to make onesies for you.