Radio Shack, before their bankruptcy, carried one. You might contact any remaining local stores and see if they still stock it. Catalog # 15-0024. Other than that, there are no retailers that carry one.
The FM filter almost always goes before the first amplifier.
The Radio Shack FM filter is not suitable for your location. It will not attenuate WFDD on 88.5 MHz because it is designed to allow reception of CH 6 TV. It does well at the high end.