How much does it cost & where can it be purchased?

As for the Essence, I agree with Adriaan, Keith, and Peter.  When I had my Essence upgraded with a Q switch, I was told (by Mica and/or Val) that the standard Essence package has the Q hardwired to always be on, to always boost the cutoff frequency.  The upgrade to add the switch gave me the option to turn off the boost.
 
I don't know about the Activator packages (or if I did, I no longer remember), so I can't speak to those.

Barry, you can contact Alembic here.  Keep in mind that the shop has been significantly inundated with an outbreak of flu.  I don't know the current price of a Q switch ordered by itself, but you can get a ballpark idea from the fact that as an addition to an Activator order, the switch is $50.

As Dave says the Q's add decibels not subtract them unless which perspective you look at it.  Leave it on then turn it off or vice versa
I like to leave them on and annoy the PA engineer!!!..Have to pad it man!!!!

Hello Shae, welcome to the club. If you can afford it, ask Mica about a 3 position Q switch. The extra versatility will yield more available sounds at your command. You won't be sorry.
Kris