Officially, the figure was called "Elizabeth Anne McSweeny" ("Buffy" is a short form of "Elizabeth", used by the late Queen Elizabeth the Queen Mother in her youth at notoriously-haunted Glamis*; "Anne" was Buffy's middle name; "McSweeny" was a name Foundry attached to a lot of their promo figures), and came in a pack with Father Eamon McSweeny, a heavily-armed priest, that was set out with all mail orders in July 2001.
Examples come up on eBay from time-to-time. Unlike many of the the other promotional models Foundry released around that time, this one hasn't been listed in the "Foundry Specials" section of their web-site.
Alternative Buffy-alikes in 28mm: Hasslefree ("Sadie"), Cool Mini or Not ("Sarah") and Nightshift Games/Team Frog ("Bambi"). Ground Zero Games does one in 25mm, Graven Images/Monolith in 40mm.
*There has got to be a game in that: perhaps a 1920s Buffy Bowes-Lyon hunting ghosts in a Cthulhu game?