You can buy the lens on Amazon or TTartisan store.

Those are reviews of the E-mount version but obviously you can expect the same performance on MFT:

Caleb writes:

How does the Lumix G9ii perform in low light situations? I took it out and tested the low light capabilities of the Lumix G9ii in a very real, everyday situation, a Christmas light walk.
I tested out several different color profiles at different ISO ranges. I think I found the limit on what the G9ii can handle.

G9II is in Stock at BHphoto, Amazon US&EU, Adorama, Fotokoch, FotoErhardt, WexUK.

DigitalCameraWorld tested the OM Digital 40-150mm f/2.8 PRO and concluded:

The M.Zuiko 40-150mm f/2.8 Pro is one of my favorite lenses, and one of the best at what it does. For photographing wildlife on safari, I simply could not be without it; it’s half the size and weight of most full-frame 70-200mm equivalents and, with an effective 300mm at the long end, offers much more reach as well. Its tiny size and weight combined with the gobsmackingly good image stabilization of Olympus / OM System bodies makes it my preferred weapon for telephoto shooting – particularly with the IP53 weather sealing and 2x teleconverter compatibility (giving it a 160-600mm effective range!). It also has the coolest lens hood I’ve ever seen!

Red35 writes:

I am here to voice my opinion on Micro Four Thirds, in particular for travels and documentary projects, base on my two decades in pro photography and being an avid traveller and a passionate photographer for almost 30 years. I have used (and still own) full frame, APS-C and M43, each has its place on my shelf but M43 is one that I consistently take out and travel with me around the world and this video explains why.

The lens is available with discount at Amazon US&EU.

I bet you forgot Olympus made this little gem camera!

The new G9II makes into the top :)