So what radio are you looking to add this for? Portable, Desktop/Mobile or RTL SDR? If it's for a base station then do you already have an outdoor antenna?
A suggestion: you might consider buying both a high and low pass filters and form a trunking band-pass filter.
Example: This one would attenuate signals up to 740mhz:
http://www.minicircuits.com/pdfs/VHF-740+.pdfThis would allow signals up to 950mhz and block everything above:
http://www.minicircuits.com/pdfs/VLFX-780.pdfSince these are SMA connectors, the good news is this will work readily on a portable scanner (at least the Uniden scanners.)
Mini-circuits also has amplifiers if you're so inclined. They're much higher quality and have excellent resale value.
An example broadband amp:
http://www.minicircuits.com/pdfs/ZFL-1000LN+.pdfHere's an 800 Mhz version:
http://www.minicircuits.com/pdfs/ZX60-0916LN+.pdf