Permanent summons are best done with Epic spells. For a non-epic solution try this: -
Permanent Summon Monster
Conjuration (Summoning)
Level: Clr 9, Sor/Wiz 9
Components: V, S. F/DF
Casting Time: 1 full round
Duration: Permanent (D)
This spell is identical to Summon Monster 6 except that the creatures remain until dismissed, dispelled or slain.
Not really all that useful for creating armies since there are lots of spells that get rid of summoned creatures by the score. A few temples and wizard guilds arming their acolytes and apprentices with wands of dispel magic can get rid of a summoned army quite rapidly. The various Magic Circle spells will keep summoned creatures away and so on.