With the P-mount system you can use any P-mount cartridge that you want.
But I would use a medium quality, one from about $40-$80.
Now for the original one that came with ithttp://www.lptunes.com/Stylus-for-Techn ... slb020.htm
TYLUS FOR TECHNICS SL-B20 SL B20 SLB20 TURNTABLE
The Technics SL-B20 SL B20 SLB20 turntable was fitted with a Panasonic Technics P-328 P328 phono cartridge and a Panasonic Technics EPS-328CS EPS328CS needle stylus. A photo of the stylus replacement is supplied above. The color of the stylus varies. Please verify and use this information as a guide only since the cartridge and stylus may have been changed through the years.