What are possible alternatives for a depletion PMOS
There are extremely few situations where a depletion mode MOSFET is necessary or preferred. It is extremely unlikely that you happen to have an application where you need to use such a device.

There is no reason to think that a depletion mode device will be as fast reacting as alternative more modern components. If you want it to act as a switch then you will need to hold it off until it needs to be on, and there is no reason that off to on speed of depletion mode device is especially fast compared to alternatives 

Achieving fast reacting" circuits is 'just a matter of design'.
If you describe your requirement (rather than an assumed solution) in detail we will be able to assist you with a good solution using readily available components. 

What order of switching speed do you require?
Please tell us as much as possible about your application.