Etsy is the first thing that came to mind as well.
I found a shirt for my 11 year old SD at Children's Place that says "Best Big Sister" not sure if Children's Place has anything like that for your son? Worth checking out.
Edited to add ..."Café Press" is similar to Zazzle and you can custom make shirts on their site as well.
Me: 31 & Husband: 40
Married: November 2014
Re: Gender reveal ideas???? HELP PLEASE
as for the onesie, I'm not sure. sorry.
Married: October 2014
TTC #1 since September 2015
I agree, check out Etsy and Pinterest!