Need to purchase a separate ground cable - from my TT to my Phono Preamp

Any basic wire can be used for a ground between the TT and the phono preamp.  I have made some with spade termination and have used some with just raw wire terminations.  It is not in the signal path. 

Generic hardware store copper wire is fine.

If you don’t want to make one Audioquest makes a inexpensive ground cable that you can buy off Amazon…

Any bit of metal wire will work - light cord or mains - outside the audio loop but eliminates hum if there’s an earthing issue 

But make sure it's been cryogenically treated, reviewed by at least two fan mags (or the equivalent online vlog), and the lugs are rhodium-plated unobtanium!