Does anyone know if there is a company

Standard size 300s seem to all be 96" long, but any company that makes tanks can make you a custom tank of that size. You will just have to pay more to have it made to your dimensions.
I've never seen a 300 gallon rimless. You're talking about some serious weight there. I don't think its an off the shelf thing, its going to be custom, and very, very, very pricey
Just off the top of my head...

Deap Sea Aquatics
Aquarium Design Group
Miracles Aquariums
Coast To Coast

And on the low end, Glass Cages.

You're looking at several thousand smackers for a custom rimless 300.