I wouldn't say they are common.

MEC was the only co-op I have ever been part of. I'm pretty sure they stopped being a co-op and sold it to private equity.

For the Americans in the audience: MEC ~ Canadian REI.

See perhaps: https://en.wikipedia.org/wiki/Consumers%27_co-operative

Vanguard is "kind of" like a co-op, in that it is owned by its mutual funds which are owned by its customers.

Fidelity is still better in some metrics, however.