I have Walls brand coveralls. I buy them right from their web site. I have a pair right now going on 7 years old, so the money spent is worth it.
I recommend finding them locally and trying them on to find the best size - Farm and Fleet around here. Once you have them on, raise your arms above your head and see if the crotch cinches up to your nads. You want them "baggy". You'll be more all around comfortable.
I wear them primarily in the cooler months with very little on under them, weather permitting. Winter time I have sweat pants under them and maybe a thermal shirt.
Summer time, 90 degrees, nope, just shorts and a tee shirt. Don't mind getting dirty. Actually the shower after wards is welcomed.