They are going to be expensive, best advice is keep watching ebay for a good used one or possibly look for a bone yard that has one.
Or just pony up the money for a new one
That cap did fit many of the bike models over the lat 60s 70's same cap is on my 360 and my 450 so looking just specifically at a 350 model is limiting. Look the part number up in an online parts fiche, should let you see the different models it came on. Hell you might get lucky and find a Honda shop that still has one or two.