Yes and no.
I think that homebrewers typically have to go through homebrew retailers while breweries can go to the manufacturer.
For example, yeast. Homebrewers have access to the same yeast commercial brewers do from Wyeast, White Labs and the various dry yeast labs. But I buy from my homebrew shop and breweries probably buy from the lab.
Grains and malts probably come from the same place and there seem to be online retailers that sell to both homebrewers and commercial breweries. But commercial breweries buy this stuff by the ton, so they clearly get it from a different retailer or wholesaler, even if it's the same stuff homebrewers can get. Hops are probably the same way.
I'm guessing that most malts, grains, hops and yeast come from just a few producers that sell to both commercial breweries and homebrewers... the difference is the retailer.