A friend and I placed an order through a deputy service called "Crescent Shop" for a bunch of parts from Sanwa. They seemed friendly, able, and the fees weren't unreasonable. There are also people floating around this forum who do periodic Sanwa orders for joysticks and conventional buttons, and will take orders for special parts.
From what I have gathered, the Sanwa buttons are the exact parts Konami uses, and Sanwa will sell them in quantity 1. Konami has certainly been known to frequently use other Sanwa parts in Bemani cabinets (IIDX and DDR both use parts distributed by Sanwa Denshi all over the place). If you order the Sanwa buttons, you're also likely to get the exact microswitch used by pop'n music, as well. I will say that the 80gf cherries commonly used by ASC builders here in the USA are not at all an inaccurate approximation - I can't really tell that much of a difference, especially given the game mechanics of pop'n music. Now, the 200-500gf microswitches that come with some buttons are noticably harder to trigger and feel "wrong" compared to the arcade (of which I played several in Japan).
For reference, I play on a Ransai ASC. I'm not sure he's making them anymore, but they were at the time widely considered the absolute best pop'n ASC out there, and still hold their own with the likes of Desktop Arcade. It does not use Happ buttons (I'm not sure where they were sourced from, but they appear identical to the arcade buttons in every way), but it does use Cherry microswitches purchased from Happ.
gf does in fact mean "grams force". I guess I'm just always used to putting that on there (engineering background). 1gf is the weight of a 1 gram object at sea level on earth. The f is put on there to distinguish it from the gram mass unit. There is, accordingly also a "pound mass" (lbm) that is the mass of an object weighing 1 pound at sea level on earth.