Start calling motorcycle shops, battery stores, and every other place that sells vehicle batteries listed in your local Yellow Pages for a YB14L-B2 or equivalent. Expect to pay about $60. Or, as said above, buy a cheaper one and make some modifications to the battery box. Adding longer cables that cross to terminals on the other side of the battery should be easy enough for many of us.
The recommended capacity is 190 Cold Cranking Amps. Here are the specs that should match any brand of battery:
http://www.interstatebatteries.com/cs_eStore/Products/RT/PID-YB14L-B2(MotorCy...By the way, the Big Crank battery is not a dry cell. It is a sealed absorbed glass mat (AGM) lead acid battery. These are spill proof, possibly more resistant to vibration damage, and never require replenishment of distilled water.
Batteries are dated, and the general rule is to avoid any over 6 months old, and try for one no more than 3 months old, but...but many motorcycle batteries are shipped without the acid which is added when sold, and they don't start to age until the acid is added.