I have seen a few threads about onesies and sleepers and places to find them online so far the only back zipper ones are made by Privatina.eu I hope I spelled that right any-who.
I know that if you live in Europe it is relatively easy to purchase from Privatina.eu over there but in U.S they cost almost 290 dollars to purchase and ship from Europe.
I found a website that makes adaptive clothing like Easy Access Products and clothing for People with Special care needs, physically handicapped and disabled which has been posted many times on here recently.
This company Adaptations by Adrian is based in California and they sell back zipper onesies and back zipper full body pj's with relatively childish prints which are rare to find in that style as far as i know on the internet.
Note i have not ordered from Adaptations by Adrian before, they have a contact email and facebook page as well to check out if anyone is interested.
hopefully this helps someone out in there clothing search which is an almost always difficult search.
This Thread is Very wordy and long sorry about that