In most older synths, the battery is often a mercury type that has to be soldered into place by a tech. I doubt there are many, if ANY, that let you slip another 9v into the slot! If you're due for a trip to the shop anyway, have them replace the battery as a matter of course and keep track of the date. They're slated at about 3 years, but I've had them go for 6.
If YOU are that clever and trust yourself, do a search for electronics parts houses. Korg has to buy batteries in bulk from somebody just like RCA does.
In any case, its probably something for a tech to do. Remember that these guys get $35/hr. and up just to one the thing, so BE EXACT about what you want done and where any problems lie. Makes their job easier, too.
Huey