Any motorcycle shop worth the name will have or get a battery. Some battery shops (try yellow pages or online search) will have them, just look up the battery number you want. Yuasa has the YBL14L-A2 Yumicron, 12N14-3A standard, or even the YTX14AHL-BS maintenance free type.
For acid filled batteries make sure the store fills the battery to the full line on each cell and that you charge it at low rate (1A) for a couple of hours before using it. Officially you should top it up to the full line with acid after this first charge, but in practice this is basically never done (since you have the battery at home and no box of acid to top it up with).