What manufacturer doesn't release offsets? Sounds like a crappy manufacturer? Any respectable manufacturer will release the offset to the original owner. Are you not the original owner of the wheels. If that's the case, I can see them not releasing the wheels as they might not know exactly what it is.
09 E90 M3 - Interlagos Blue / Black Novillo / Sycamore Anthracite / Eibach / HRE