The Cambridge Crystallographic Database will give you the ability to find any pyridine-Mn bond lengths. This will also give you a set of references with discussions, and that you will also be able to follow through the Science Citation Index, etc.
What oxidation state and what other ligands are you interested in? Those factors will have huge effects on the bonding. I would recommend using the CCD as Neil suggested, but be careful to separated the complexes out by oxidation state.