Here is a particularly apt description of Haw Flakes (local copy). Even more importantly, here you can see what Haw Flakes and their packaging look like (local copy). (Apparently, lots of people recognize them by the fact that their packaging makes them look like firecrackers.)
Haw flakes are mistaken for drugs (local copy), used as (tasty!) packing material (local copy), given up as offerings (local copy), immortalized in the names of songs, the fourth best thing in the world (local copy), an easy way to cheat parking meters (though I don't recommend you try this) (local copy), and banned by the FDA (local copy).
Additionally, we are given to understand that Haw Flakes are non-judging, non-phlegm-flinging, and non-smelly (local copy).
One theory on the composition of Haw Flakes is that they are made from the berries of the hawthorn bush (local copy). This would be consistent with what John Lee's mom told me a long while ago - "they're made from a kind of berry."
Most Asian or `international' groceries will have them. In particular, here are places they have been attested to exist: