Dickies makes coveralls fairly big, you might be able to find some locally to try on? No place around here sells 'em that I know of, but then I am kinda in the sticks. If $$$ isn’t an issue, maybe Gibson and Barnes makes custom large-size suits?
We used to have a member that was 6’5 and was about 350 and he wore Walls brand coveralls. They come in Khaki, Dark Gray, Navy and more. They have the slanted zipper chest pockets and the back pleating, they have a heavy substantial fabric and they go up to a 6x
Search KH Walls on google. also Rothcos go up to 6x
See I just recently started looking for a flight suit andi run in the same problem. I’m 6ft4 and weigh about 325lbs and i found the only suit that would fit me we the rothco flight suit and that was in a 6XL and it was black.
Not the best site to get them on because you need to order I think at least 2 or $75 worth. But…These look good. I am currently waiting on a 5xl Navy Blue (of course they only offer certain colors in certain sizes) I like this because of the loop on the chest where you can hang a flash light or Spirit/Walmart/Rubies PKE Meter and there is also a cell phone pocket in the front which makes it more modern.
Red Kap is another option.. similar stuff to Dickies and Walls. Some shirts go up to 6XL Tall. Coveralls are pretty good and I think they get upwards to 5X or 6X.