Jim,
you can try a boat electrical cable dealer. They sell small lengths of the heavier tinned copper cable suitable for starters and battery systems on boats - both positive and ground/neg, so red and black sheathed.
Another source is an electrical switchboard manufacturer. They use flexible cables to connect into the physically smaller devices so popular nowdays in switchboards. They will tend to have plain copper flexible cables and as an added bonus should be able to make the cable with crimped lugs to length.
Switchboard manufacturers usually have first class cable, not the shitty stuff so often found online.
I would suggest a heatshrink section, complete with internal glue be used over end of cable/lug to seal the crimp area. Not absolutely necessary as it will outlast us all if good quality, correctly sized cable used and crimped properly.