There are some on aliexpress, you may have seen those already. I would suspect they are relatively custom, as they tend to be epoxied on to the motor shaft.
No I hadn't been able to find anything. But searching again now I came across one store, which then leads into many more as always on aliexpress.
Pretty much found what I am after this time:
https://www.aliexpress.com/item/32817651811.htmlIt'll be easy to turn up a mount for it.
Most people are not going to be assembling motors from parts off digikey IMO. They'd either buy the motor fully assembled with motion control, or add an encoder to an existing motor (CUI encoder or AMS IC). Both of those options are a magnitude more expensive in volume, however if its a one off, they will be cheaper.
If you don't care about absolute positioning, and want low cost, what about sensorless?
https://www.digikey.com/en/articles/controlling-sensorless-bldc-motors-via-back-emf
As much as it may seem odd, but repairing and modifying motors is quite common. The motors themselves can be expense, but often there's also quite a lot of work in adapting a new motor to fit the machine it comes off. True probably not digikey, but there's mcmaster carr etc.
For the purpose of commutation (FOC control), 3 hall effect sensors and a magnet don't get much simpler which is why it's still almost universal. Often paired with a high-res optical encoder for servo control.
Sensorless is great, but much harder if you also need full torque at low rpm. Quite common on fans, centrifugal pumps etc where the starting torque is low.