I feel I shouldn't "suggest" as such but refer to the one I bought! Works very well, but remembering of course such factors as line of sight to transmission, capabilities of receiver/scanner etc. all affect performance.
Sky Scan Mobile Scanner Antenna : Scanner Accessories : Maplin
My wife's uncle (RIP) was a great radio ham and as HD mentions, made his own aerials which were customised to the job and out-performed anything to be bought ready made, and cheaper.
You pays yer money and takes yer choice ...... as they say.