A flea market. Find a local leather worker and they can make one super fast with good quality leather. "Genuine Leather" is one of the lowest quality leathers out there.

You'll probably need to take care of it if you want it for work. Cleaning and conditioning it.

Personally, I avoid belts that are narrow, like 1" wide, as I think they like to twist more if they have to do a lot of lifting.

I bought mine at artisan fairs, from leatherworkers. Been wearing it for more than five years now every day. It shows a bit of wear, but I think it will last at least another five

Best advice is to learn how to identify real leather vs laminated shit. Once you can tell by sight, smell, feel you can thrift great items. Also going to markets and craft fairs, you can stumble across artisan leather makers (but even then you gotta make sure they are using quality products)

I've heard good things about NAB Leather, but have no personal experience.

What you want is single-piece full-grain leather about 10-12 oz weight. (This is a leather thickness measurement, not the weight of the belt.)

I find that trade shows (e.g, home/garden shows) often have a few vendors who make top quality belts.

I found my belt at a secondhand store. It did take some hunting before one turned up, but I can change the buckle on it if I want. I got it for extra cheap because it had snaps holding the buckle on originally, but one of the snaps was broken. It was an easy enough fix to remove the snaps and replace them with chicago screws, and it's honestly a bit more durable with screws anyways.