After uncovering a dead Zap Stop (improperly mounted in electrical
tape, vibration had broken both leads off it) in a friend's boat, I went looking into them. You CAN indeed buy the same type and rating of diode for about three bucks from a parts
place, quantity one, if you know where to order what. Typically plus $5 shipping
and handling if that's the only thing, so we're up to eight or ten bucks.
Which also makes it not unreasonable that the same thing sells for $25 on the chandlery
shelf, marked up by a wholesaler and retailer with instructions and bits of shrink wrap, etc. for a sailor.