Pelican has the reference sensor for $163 - which is kinda high.
http://www.pelicanparts.com/catalog/shopcart/944M/POR_944M_FULinj_pg2.htm#item5
The best deal I've seen is at Lindsey Racing - $72.95 for a generic Bosch sensor! (which is exactly the same as the OEM Porsche part) -
http://www.lindseyracing.com/mm5/merchant.mvc?Screen=PROD&Store_Code=LR&Product_Code=944-606-115-00&Category_Code=944ELECTRICAL
The speed and reference sensors are both the same.
Used, I've bought them for as little as $25 (from parts cars).