Echoing a bit with the others above, until the end. I'll mark it with ***
If you love the M2, and are going to spend the money to get a shiny new shaft, get a fitting. I cannot stress that enough.
I would certainly recommend getting it all done at the fitters. You don't have to buy it all there, but even if it cost $200 more, the confidence and (just as important) the reassurance that you have the right club and shaft is worth it.
I say this out of experience. I've spent probably $200 and countless hours just trying and returning clubs over the last two years, when I should have just bit the bullet and went in for a fitting.
Now that I have, my driver is the straightest club in my bag. Saving up for an iron/wedge/putter fitting next.
***
All that said... if you are looking at saving a bit of dough, you'll probably want to get more statistics. Most courses have access to a Trackman. You'll want to get as much data as possible (your attack angle, in to out, ball trajectory, stats on your speed [you state you are ≈ 100 mph, is that out of 10 swings? 100?]). Once you have all of that juicy data, you could probably post it and ask the same question on reputable sites such as this one as well as reddit and (if you want to get beat up a bit), post it on golfwrx .
You could probably give all that data to a fitter, and without the cost of a fitting, they could point you in the direction of the right shaft for your M2. They probably even have used shafts.
Cheers.
Also, where do you live? I'll bet we can find you an excellent place to get advice, trackman, etc. on these boards.