Apart from searching Yahoo Japan Auctions, you're pretty much out of luck. Every time I've searched for Tsukihime (usually Tsukibako) there, I've always found multiple sellers. It's the price that'll get you (starting in the
,000 yen ranging to 25,000 yen... I've seen some sell for around 13,000 yen though...it's all very relative.
(I spent an embarrassing amount on my copy, I probably could've got it much cheaper.)