http://www.mypoolpal.com/
They have an online store at that site, and some of their products are also carried elsewhere online and presumably In Real Life as well. While an extra small special needs swim diaper labeled 6/8 or extra small doesn't _look_ like a 5/5T, in practice, it basically is.
I can't tell you how happy I was to discover this. Their special needs section includes adult sizes as well, if perchance you are reading this and thinking that 5T still counts as little and wondering what to do about some other wonderful person in your life whose pool time is problematic.