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 and Wizards at 17th level.
Finally, Planar Ally, a 6th level spell available only to Clerics, can summon an Invisible Stalker if your otherworldly entity of choice deems it appropriate.
Note that neither Gate nor Planar Ally place any restrictions on the creature's action, and from the description of the Invisible Stalker it probably won't be too happy to help you. The combination of Magic Circle and Planar Binding can be used to make it serve you, though.
Outside of these options, it's important to remember that the Monster Manual is intended as a DM resource. The description of the Invisible Stalker is meant to provide you with potential story hooks. The sentence you're asking about could probably be rephrased as:
An Invisible Stalker makes a great servant for a wizard or sorcerer NPC.
(Not actually a quote.)