There’s a range of cheap (< $20) USB Digital TV receivers that due to their wide tuning range can be used as Software Defined Radios (SDR) when combined with other software to tune and receive the signal from the dongle.
RTL chipset dongles are supported by open source software rtl-sdr. Download the source to your Pi and build following the instructions on their website. By running as follows:
rtl_tcp -a your_ip_address
you can stream the data received from the dongle inserted into your Pi to other software running elsewhere (your desktop/laptop) like SDR Sharp (by pointing it to the IP address of the Pi) which you can use to control the tuning of the dongle.