You can buy a mob spawner at spawn for $150,00 by right clicking on the sign. You can use them by placing them and right clicking them with a spawn egg. Just specific mobs are allowed to be used with spawners. Spawn eggs can be obtained by hitting a mob with an egg (Plugin EggCatcher). An egg thrown by a Dispenser does not work.
please expand that list if possible
Just a few mobs can be used on spawners, if you try to set a spawner with a disallowed mob, it will result into one mob getting spawned and the spawner destroyed. This is enforced by the NoSpawnerChange plugin.