what type of manifold, intake, exhaust, cast iron , aluminum?
if it is a exhaust manifold and cast iron any local welding shop
in your area should be able to fix it, but don't expect it to last plan on replacing it. cast iron exhaust manifolds become brittle over time you can weld it with nickle rod but with it getting hot and cold over and over, expanding and contracting it will eventually crack again. I'm a welder You can pm me with a little info and I can send you in the right direction.