I have seen solid color Onesies on several internet sites.
Baby Pants
https://www.baby-pants.com/onezie.php
BigBabyBums at Etsy
Adult Onsie
Protex
Blue Onesie
AdultCLothDiaper
"T" Onesie
BabyKins
Adult Onesies
And more:
The problem is not availability of solid color Onesies rather the price. All of these outlets are Retail sellers and their prices are too high to provide a decent mark-up for you as a fledgling business.
Here are some ideas to help make that happen.
- Create a business entity with a unique name such as "My First Business".
- Get a business license so that you can buy at wholesale price.
- Obtain any other permits and licenses you need to become an aftermarket reseller.
- You need to learn where other sellers are buying those Onesies wholesale and then do that yourself.
- An industry convention is a typical way to get such contacts.
Cold calling these resellers may yield a wholesale source for you if the company is a small (and friendly) outfit such as yourself and does not feel threatened by competition.
As a startup business you can get away (for a while) without having the necessary licenses and permits or even a physical site, however you will have to buy your inventory retail until you make the right contacts for a wholesale seller of the goods you wish to resell.
If you do make enough income to worry about taxes than you should have all of the legalities already done.
You might consider buying an established "Tee" shirt printer/seller business that is struggling or just tired of it. That way all of the groundwork has been done. You would get:
Licenses
Tee shirt printer
Computer with special graphics and printer software
Inventory of conventional Tee shirts.
Perhaps a lease on a physical site or possibly some sort of portable business.
Another alternative is to establish a business relationship with a Tee shirt printer shop and have them do the printing for you and then return ship the product. That way you would not have to purchase expensive silk screen equipment or other printing devices.
Good luck, I hope this helps you.