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.
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.
For the Americans in the audience: MEC ~ Canadian REI.
See perhaps: https://en.wikipedia.org/wiki/Consumers%27_co-operative