I came across a unique and relatively low cost fisheye lens, while searching through B&H Video's selection of goodies for the Alpha-mount. The BOWER MF 8mm f/3.5 "Fisheye"
An interesting lens, in the sense that it goes almost straight to infinity focus after the subject gets to ten-feet.
All Manual (focus and aperture (3.5-22) with NO electronic connection at all, so you probably cannot use it on the A900 (as it does not like MANUAL lenses at all.)
For $349 ... it is a low cost entry into the "ALL or NOTHING" photography aspect. Could be a fun lens for a party or one heck of a tight room.