Molar absorption coefficient of H2SO4(aq) for UV-vis spectrophotometry
I never ran into uv absorption data for (aqueous) sulfuric acid and I doubt that there is any absorption at $\lambda$ = 508 nm. 

There is an old paper by K. F. Palmer and D. Williams in Appl. Optics, 1975,14, 208-219 (DOI) on the optical properties of sulfuric acid droplets with respects to the composition of clouds on Venus and a more recent one from the group of J. B. Burkholder in Phys. Chem. A, 2003, 107, 1112-1118
(DOI) on Vibrational and Electronic Spectroscopy of Sulfuric Acid Vapor. Both, while interesting and therefore mentioned here, are not relevant for your experiment.

My suggestion is simple: 

Measure your ferroin sample in aqueous sulfuric acid (or whatever solvent is required) and use the same solvent as a blank. Most spectrometers will automatically perform the subtraction of the blank from your sample!

Concerning a (free) source of UV-vis absorption data, there is the NIST Chemistry WebBook.