You need an attorney. Then your attorney and his investigators need to try to track down witnesses. See if there are any home or business security cameras on the scene, etc. Otherwise, you won't be able to prove it. Also, was anyone taping the scene? Okay, so if you can get the evidence, you might have a case.
There is no way you will get anyone to take a lie detector test. If you got the guts, file a complaint with internal affairs. Rresearch if there are any organziations in your city that take on the cops for brutality.
Also, if have any sense you would already be collecting evidence. EVERY police taser that shoots darts also deposits little id labels all over the place. You might even have some on your clothing. EVERY time a police taser is used, it has to be logged.

