I bought this antenna (comes in a pack of 2!) on Amazon for $19.95, and it works wonderfully with my BCD996P2 scanner! It is intended for wifi but it works great for back-of-the-set scanner use. It is bringing in 700 MHz / 800 MHz trunked systems better than my roof-mounted antenna, and performs acceptably well for VHF / UHF reception too. It comes with 10' of cable but you can simply leave it coiled up and park the antenna with its mag mount on top of the scanner, or you can extend out the cable and place the antenna in a more optimal location. It ends with an SMA connector so depending on your scanner, you may need an SMA male to BNC female adapter.
This sweet indoor antenna is the best $20 (again, you get two antennas!) I have spent in a good while.
This sweet indoor antenna is the best $20 (again, you get two antennas!) I have spent in a good while.