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

YO MR. CRUSTYCOOT - I am going your route - Vegan Wire with a dash of hot sauce - Combined with a slather of pig fat so it doesn't get dried out. 

20 ga. with with spade lugs.

Thank you to all and to all a goodnight.....

My TT has no separate ground wire and it is dead silent. 
even with a MC cart. 

Electrons have become very fashion conscious lately...conductors must be silver plated, have at least 6 Nines, clad in PTFE jackets, with gluten-free packaging.

In other words, standard 20 ga. hookup wire with or without spade lugs crimped on should get the hum out.

GOT IT 

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!

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

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

Generic hardware store copper wire is fine.

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. 

when I made mods to segregate ground cable from Technics 1200 MK2, I just used normal black piece of wire terminated with spade all found in my tool bins. My signal tonearm wires are vintage NOS Ortofon 26AWG