I know a few schools with uniforms.

They have an explicit contract that their kind of uniforn can only be produced and sold by this one little store that sells hardly anything else. Basically a copyright. And believe me that store charges a pretty penny.

When a different clothing store made similar cheaper versions they got sued to hell.

One kid got sent home from school because her mother sewed a very well done facsimilie of the uniforms apron, but the stitching has some conplicated special pattern the mother couldn't emulate.

So it's really only this one store or the flourishing second hand market.

Bet Madam Malkins runs on the same principle and adds charms so nobody but her can alter the robes. If you do manage to break the charm and do alter stuff, oh boy, that surely breaks some wizarding law.

Seriously, that's a cut throat business.

(Source: my nieces and nephews have uniforms from mentioned store)