Rosenkreuzstilette was originally released as a pressed disc in December of 2007. It has since been re-released as a digital download at DLsite and Melonbooks. The game costs 1,260 yen (about $13.50 US). The original pressing is relatively hard to come by, so you're better off buying the re-release (no brokerage fees, taxes, or shipping).
Incidentally, Darkside Translations and Zeppy released an English localization of the game last July. Installers have been made for both versions of the game ("Original Pressing" for the disc, "[erka:es] The Best" for the re-release).
Links --- RKS @ DLsite (English Navigation - just click "Maniax" to skip the splash page) - http://eng.dlsite.com/work/=/product_id/RE051185.html