Anti-polygraph has given you good advice. Steer clear of the FBI polygraph, if you fail it will follow you to every intelligence agency. This historically has not true for polygraphs at other law enforcement agencies like the Secret Service, but this appears to be changing with the advent of Scattered Castles and other federal databases storing polygraph results.
And, not to throw water on your opportunity, but you only think it is your dream job. The FBI told me an offer was coming for a position when I was a GS-14, and further told me not to accept offers from other federal agencies. What was their offer? A GS-14 (lateral - no promotion to a GS-15, no moving costs paid from DC to Redstone Arsenal AL, and no signing bonus.) I passed. Got a GS-15 a few months later in DC where the job market is much better.
There is a certain allure to being in the intelligence community, but taking a poly from any agency is not worth the potential damage to your reputation. Remember, if the intelligence agencies were any good, the Chinese would not have J-19 stealth fighters and our secrets would not be walking out the door on a daily basis. Our adversaries are laughing from here to the Ho Chi Minh Trail.
Also, there is no “preparing” for a polygraph. The test is not valid.