I seek a highly specular reflective (>99%) surface for UVC (~253.7 nm) photons. Is such a surface possible? Does the answer change with the angle of incidence? If specular reflectance changes with angle of incidence, what is the relationship between them?  If such surfaces are possible, where can I find a source?
 A: Your question seems to have two parts.
Can you get the reflectivity > 99% The answer is yes. Dielectric mirror stacks can give you that.
Does the reflectivity depend on angle of incidence The answer is also yes, and also you may want to consider polarization.
As an example this is a mirror designed for the 4th harmonic of a Yb laser. It is a dielectric mirror with high reflectivity and it is designed for the mirror to be placed at a 45 degree angle with respect to the incoming beam. for your wavelength both the S and P are probably greater than 50%. It is about 200 dollars, and probably not that large probably 1 inch in diameter.
If you don't have this mirror at 45 degrees then it is a little more complicated, you can order a mirror for the angle you want, or you can consider that as you change the angle of the mirror the effective length in the dielectric stack is changing the path length in the stack. This effectively shifts the reflectance spectrum with the angle. Something like 1/cos($\theta$). So for small angles it doesn't shift much.

If you want to have >99% reflection from a wide range of angles the it more difficult. If the angles start to large you will be outside the bandpass of the dielectric mirror.
Pure Aluminum  won't get you the 99% but is probably the best compromise for something that can be large area to get a specular reflection and it can shaped, for example by diamond turning to get a specular reflection. And as pointed out in the comments it can be evaporated. It will have a pretty flat curve for reflectivity as a function of angle.
However Aluminum will oxidize over time and lose its reflectivity some and have increased scattering. Most optics companies will recommend a protective coating and those may be a few layers of dielectrics. This brings you back down to the 90 ish percent range.
