The cheapest way seems to be harvesting them from actual keyboards. It's incredibly stupid, but they're just expensive in small quantities.
Greens seem to be blues with a different spring. I opened one up and it could be the spring used in blacks; I'll elaborate on this as soon as my Cherry G80-3000 with black MX arrives (which was somewhat cheap), but judging from pictures comparing the two, it does look suspiciously like the spring used in blacks.
-huha