There's no slimes in the swamp of garama, Which is strange since in vanilla MC, Slimes spawn in swamps.
Therefor i suggest that the swamp biome in garama, And the marsh biome in ascalon have an increased slime spawning rate.
Keep in mind that slime spawning directly corresponds to the cycle of the moon. For the best chances of finding some I'd recommend you wait at the largest, darkest patch of land you can find in a swamp during the full moon.
Well on multiplayer servers it's not nearly as good but you should at least be able to collect a stack of slimeballs during a full moon. Also, a new moon will never spawn any slimes so don't waste your time then.