Rather than converting the lens to M mount, consider buying a 7 or 7s that will mount the lens as built. The 7s is a beutiful camera, built like the proverbial tank. The 7 is slightly less, IMHO, but not noticeably, and much easier to find. Canon sold a lot of them. For the price of the conversion, you can easily find a very decent 7, and still have an original condition lens.
As to scarcity, it's not common, but easily found, if you have the ~$600 asking price for most. Be aware the TV version is different in several respects, and has no RF coupling.... the cam is not present, and the rear element is not cut to allow mounting one. They sell for less, and could be problematic.
Harry