Any restaurant supply website has them, just Google. You likely have a local bricks & mortar restaurant supply place that has them too. Or Sur La Table sells some very nice ones - a bit nicer than the ones typically in restaurant supply stores - for something like 3X the price.

Sam's Club has lots of restaurant supplies. I have bought big stainless bowls from there, and a nearby discount mall has a couple of stores that sell big heavy stainless bowls.
Also you can buy on line from Food Service Warehouse....

Here is a link that might be useful: bowls

I found some great heavy stainless ones at a local flea market.

I've ordered some items from the company at the link (Tundra Specialties, in Denver). Good stuff, reasonable prices, quick service, and I'll use them again. I did NOT order any mixing bowls so I can't attest to which ones are good, but I've been pleased with their stuff so far.

Lehman's (lehmans.com) also has some heavy duty kitchen stuff. Everything I've ever ordered from them has been of good quality.

Here is a link that might be useful: Tundra Specialties

Based on what I've seen in restaurant supply stores, there are lots of things that are perfectly usable and durable, but not as nice as stuff sold for home use. Because who cares how nice something is in a restaurant kitchen, as long as it works. I don't know, but I'd guess that stainless bowls would be a case where "restaurant quality" wouldn't necessarily be "best quality."

I'm thinking you'd do well at any home goods store.

+1 on the Sam's club (or Costco, or BJ's) if you belong to any, that's the first place I'd try.

Someone here once gave some great advice about the restaurant-supply bowls that come in a dozen sizes and nest nicely. Do NOT get all the sizes, instead get bowls perhaps 2 sizes apart, and get more than one of the most commonly used bowls. Reason? When the bowls nest tightly, they are a PITA ass to reach in and just get one of the middle sizes. If they are not too closely nested, it's a snap to get one of the middle ones out of the nest.

One of my two favorite mixing bowls is an 8qt Vollrath, model 6908 that I found at a rummage sale. I'm supposing that this is the same bowl...it's '69080'. Love this bowl & use it almost daily. (My other fav it a 5 qt melamine bowl w/ the non-slip rubber ring on the bottom; it gets used multiple times daily.)

Oh, the link is for product info only; I've never ordered from that place so can't vouch for the store.

Here is a link that might be useful: Vollrath

I don't have an answer for you, but wanted to share that the best stainless mixing bowls that I use all the time were from a housewarming party I had when I first moved out in 1973. My BFF from high school gave me a hand-held electric mixer that just went belly up last year, and a set of three nesting SS bowls.

I love them and use them more often than the set I got a few years ago from Costco.

Smiles,
Sooz

I love this batter bowl. I have some SS bowls. I love them less. I do have one huge one I pull out for big jobs, though.

This one has a non skid bottom, isn't breakable (unless you try REALLY hard) and is light weight and pretty. Love the deep shape and the handle.

Here is a link that might be useful: Not SS option.

When I first volunteered to head receptions at church, there was only one giant SS bowl....and the rest were cheap soft plastic.
I let out the word that we needed more (that soft plastic is not approved for food service) and in the next few months, 3 more large ones appeared and 4 smaller ones. They aren't all the same heavy wear, some are lighter and cheaper.....but they all work!