There's no requirement for an endorsement for the FOI - walk in to a testing center, pay your $140-150, pass the exam, keep the piece of paper. You'll need it for the BGI, AGI, IGI and/or CFI.
But....if you have a state certificate indicating you are certified to teach K-12 or vocational courses, or if you are currently teaching at a local college/university, you do not need to take the FOI.
This annoys the (*&*^%*&^O out of me. I've been full-time faculty at a local university for 4 years, adjunct for 10 years at another and have suffered thru graduate courses in Education (which is really aggravating for an engineer). But because I'm not currently teaching, I'm required to take the FOI along with the BGI if I intend to teach ground.