How can I earn a bunch of arrowheads quickly in Psychonauts?

I've finished Lungfishopolis, blown my stash of arrowheads on the magnet-thing and a couple of psy cores, and just started the Milkman Conspiracy.

And now I'm suddenly shown a warning that says "You should have the Cobweb thing, or you won't get far in the Milkman Conspiracy." It costs 800 arrowheads. I just spent all of my arrowheads. I am sad. (And I've tried doing the Milkman Conspiracy anyway. I can do some stuff, but I need some particular items to get further in the game. Those items are, as far as I can tell, behind the cobwebs that block off some of the houses.)

I've already bought the dowsing rod and dug up about a half-dozen deep arrowheads, so that source of easy money is already partly depleted. Is there any other way to make 800 arrowheads in a relative hurry? I'm not particularly fond of grinding; I've only earned a cumulative total of around 500 arrowheads so far.

The consensus from various forum posts seems to be that the best way to collect arrowheads quickly is to use the dowsing rod. You say you've already bought that, so that's good! It may take some time to dig up 800 arrowheads' worth, but this map of Deep Arrowhead locations may be of use.

Note, I haven't verified its accuracy.