With a 100A alternator and output stud to fender relay wire, agree 8 gauge wire and a length of 12 gauge fusible link is good.

With a one wire, voltage drop in the charge wire is not compensated by remote sensing that ups alternator voltage to compensate... so the charge wire needs to handle typical current with minimal drop. If you have added non-stock accessories (eg fan, stereo, fuel pump, fans) that increase current draw, bigger wire is better. If electric load is increased, 6 gauge with 10 gauge fusible link would be better. I run this setup and has been trouble free.

AP stores rarely carry fusible link in 12 gauge or larger but "auto electric" stores generally carry at least 10 gauge.

NAPA also carries "maxi fuses" in high amperage ratings. If you use the fuse in place of fusible link, suggest at least a 125 A fuse with 100 A alternator. And get a spare for the glove box.