Not sure what you are talking about....I know lots of places where you can buy plants by the flat.
Linda C

A flat of plants is usually about 50 plants, often started from cuttings. For larger quantities and seed started plants they are sold as plugs, by the tray. Each tray may have from 144 to 375 plugs. Because these are grown in their own small pocket and not in the same soil with other plants, they are easier to handle and transplant into larger containers. Al

I know places, nurserys, that sell flats of seed started plants...no plugs..just a flat of plants...no cuttings just a flat of petunias, grown from seed..
Linda C

Yeah, me too. They're not at all uncommon here in the midwest (hey Linda, maybe we're sitting on a goldmine out here!).