Red makes a good point, some states are very anal about it. A quick check says you are Ok in PA. U.S. Scanner Laws
Stay out of NY. While they are legal here, some police might give you a hard time anyway.
A CB antenna might work, give it a try. You will probably need an adapter for the cable connection.
If you want a desicated antenna, one of these could be adapted;
ProComm JBC290M JBC290MM JBC290WC Mobile Scanner Antenna
Here is Universal's page of scanner antennas; Scanner Antennas
Look around, might be other you like.