Usually is "Japan" chilis, which are probably ripe, dried Serranos

To make your own ...
simmer consecutive batches of dried hot red chilis in hot oil until they are deep brown
Remove the chilis
Add more chilis
Until the oil is a deep red and as spicy as you want it
Save the oil

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Do this outside. The fumes are HORRIBLE

I've always assumed that they were using a "Thai" or other Asian style chile.

Josh

I doubt Serranos. Maybe Japones(Japanese) which you can find a lot in the Mexican dried chile section of many supermarkets and grow much like Thai varieties, as Josh mentioned.

Kevin

You can do it with standard dried red chili flakes you find in the spice isle. But that would take the fun out of growing them :-) Super chilis work well, the standard short hot chili plants you can buy at most nurseries.