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The spell Conjure Elemental, cast as a 6th level spell, can summon an Invisible Stalker. It's available to Druids and Wizards at 9th level, but obviously they will have to wait until 11th level to cast it as a 6th level spell. If you know the name of an Invisible Stalker, you could also summon it with Gate, a 9th level spell available to Clerics, Sorcerers ...
An Invisible Stalker can be summoned by casting Conjure Elemental using a 6th-level or higher spell slot. Planar Binding also has the potential to summon one if the primordial being the PCs are trying to beseech decides to send one to help.
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