Core rulebook FAQ
http://paizo.com/paizo/faq/v5748nruor1fm#v5748eaic9utx
Improved Familiars: There are various ways for characters other than arcane spellcasters to gain familiars at this point, and some of those options even grant Improved Familiar as a bonus feat, but technically each Improved Familiar option requires a certain arcane spellcaster level to take it. Does that mean that non-arcane characters with Improved Familiar have a dead feature? How does it work? If it does work, can I take an Improved Familiar as some kind of variant familiar or a temporary familiar like the occultist’s soulbound puppet?
The Improved Familiar description was written back when only arcane spellcasters could have familiars, and it wasn’t sufficiently future-proofed. To that end, you can always substitute your effective wizard level for the purpose of determining your familiar’s abilities for “arcane spellcaster level” to determine the available improved familiars for your character. In general, you can take Improved Familiars for class-granted variant familiars like a shaman’s spirit animal, with a few exceptions: First, temporary familiars like the occultist’s soulbound puppet can’t become Improved Familiars from the Improved Familiar feat, and those class features don’t qualify you to take the Improved Familiar feat.
Second, tumor familiars, as lumps of flesh in the shape of animals, can’t become Improved Familiars. In other cases, treat Improved Familiar as if it was an archetype to see if it stacks with other familiar options: since the two things it alters from a regular familiar are that it removes the ability to speak with animals of its kind and it prevents changing the creature type for non-animals, you couldn’t make a familiar that changes the creature type of non-animals or alters or removes speak with animals of its kind an Improved Familiar.