I got mine from RDS. Sold through Northern Tool Co. or can order direct from them I think and have delivered to you or an installer.
I actually have a combo 40 gallon fuel tank/toolbox but I believe they make straight tool boxes too. Many on this forum have RDS boxes. No complaints on mine after 2 years. I don't think you will beat their prices. Take a look at the RDS site as they have a zillion different boxes.
My box sits just below the bedrails so I am still able to use a tonneau cover.
If doing over, I would get their 60 gallon/toolbox combo. Same footprint but with a smaller toolbox section.
I have heard, but not confirmed, that RDS will custom build a box for you.
Tim