Adam, the problem is quite simple--Herters is out of business, noone is presently making heads, and thoses available are going at a premium--Try craigslist--Sometimes they have them. My buddy Dave had some for sale at strongsville. Maybe he can get a price to you--Don't know what he has, though.
Too bad the heads were missing. Were they broken, never put on? Kind of unusual to have that many missing heads.