http://www.atbatt.com/product/23622.asp
Note: I haven't tested that particular battery, but I have determined that the clock chip will keep time with 3v applied, and considering the original battery was 4.5v, this 3.6v battery should be a safe bet.
Also, if you don't mind the clock losing time (on the order of a few minutes a day), you can just use a standard AA. 1.5v seems like it's just barely enough to keep the chip running.
EDIT: This store has cheaper shipping:
http://www.batteryjunction.com/xl-060f-xeno-aa.html
EDIT 2: Also, Radio Shack carries an equivalent:
http://www.radioshack.com/product/index ... d=10999990