Here is what I found for a '91 300zx non-turbo on rockauto.com" TARGET="_blank">
http://rockauto.com
SPICER Part # 5051239Lower {Front Suspension Ball Joint} $24.79
MOOG Part # K9819Frt Susp; Lower {Ball Joint} $25.79
ACDELCO Part # 45D2239JOINT,FRT LWR CONT ARM BALL - {#88911729} $31.79
BECK/ARNLEY Part # 1014572Front Ball Joint, Left Lower;Complete Arm With Ball Joint;1 Per Car $169.79
BECK/ARNLEY Part # 1014573Front Ball Joint, Right Lower;Complete Arm With Ball Joint;1 Per Car $169.79
Hope this helps