A lot of people don't use exhaust manifold gaskets. The Manifolds can be filed flat (Use a good flat file with a crossing pattern) so that no gasket is required. If the manifolds were welded improperly or are warped to heck, you can use copper gaskets from ATR or special Composite gaskets.
For the other gaskets, you can look at my illustrated parts catalog and determine what part numbers you need and get the gaskets from your favorite discount GM dealer (I like Dal at Vandevere Olds) or you can go to one of the good vendors that support this BB like Cottons or Ramchargers and they can get you set up.
Local auto parts stores like O'Reilly or Advance have a good selection of gaskets also.