Is this the 10hp motor you posted about earlier? Exhaust cover gaskets are part number 301854. Try an Evinrude dealer. He can order them for you if you give him the part number.
That's why I said take him the part number. Most places won't give you the time of day without it. They probably don't have the parts book either. He knows where to get the part if he really wants to.
You can always call iboats. They work with Crowley Marine for OEM parts.