Steel / Ti frames normally have built in hangers, these are not replaceable, but can be bent back, any good LBS should have the correct tool, normally from Park, but there are several other good manufactures of bike tools out there (VAR, Cyclus etc)

Alu / carbon frames normally have removable hangers, if you have this, would initially check that it is done up tightly, as this can introduce some wobble, for replacements; unless the bike is from a brand the shop sells, don't expect them to have it in stock, but any good LBS should be able to order one for any bike from third party suppliers (Wheels Manufacturing and others).