Where can I find (to purchase) some 99% reflective mirror...?

As soon as you expose 99.99% reflective mirrors to the atmosphere, their reflectivity plummets to 95% or so. Manufacture, storage and operation needs to be in a "clean room" environment or in the vacuum of space. These mirrors are not available to the general public. I suggest that you contact your local university physics department to get some leads as to where to get what you are after.

What you are asking for is a transmissivity

t = ρ[2/(1+ρ)]² >/= 0.9999

or

r = (1 - t) = [(1 -ρ)/(1+ρ)]² < /= 0.0001 (=1/10000)

where

the ratio ρ of refractive indices of the medium (glass & coating added) to air, at normal incidence. Another pair of assumptions I made are that the coating is smoother, with 'rms' less than λ/4 at its lowest wavelength - for visible light it is λ/4 < 0.09 micron; and that it shouldn't absorb the light energy (that can be substantial only, if you point a laser beam).

Solve this set and the ' ρ ' you get is the RI of the glass you seek. then search for such a glass. This is the simplistic solution. But if you have a multi-lense complicated system this design doesn't suffice.

The best coatings in astronomical telescope mirrors are more like 95% instead of 99.99%!

Do a search for mirror aluminizing or mirror coating