Niemiec Marine in New Bedford, Mass., now has the Yanmar parts franchise, at least for east of the Mississippi. Call them and tell them what you've got. They will select the right impeller from among the many (dozens?) of Yanmar applications (raw water vs closed system, size, age, etc., etc.) and ship it to you promptly. Used to be an outlet in Michigan, too, but they sold or sent their business to Niemiec.
Well, I just saw you're in Washington. If you don't know the Yanmar franchise out there, I'd still call Niemiec. I may be wrong about the MIssissippi boundary, or otherwise I'm sure they'd know who has your parts.
Also, a few or maybe many years ago, I bought a Yanmar spares kit for the engine in my 356. It had a number of good items in a waterproof box, including the impeller and gasket and lube. I used that once and it worked swell. I've since replaced as part of maintenance with one I got from Niemiec.