Try Radio Shack for the diode , try West Marine for the 8 gauge wire.
In electrics if you can use a larger gauge wire, especially from a charging source, you help out the charging device ie alt., by reducing it's "demand load", thus making it work not as hard for the same benefit.
Also the way a 12v DC system works--simplified--power out wire needs to be the same size as power ground at least---power ground can be a heavier gauge--definitely not smaller--think of it as plumbing--the bigger the opening the higher flow---power flowing out needs to be matched, + needs to be matched to an equal or larger return source - , really does help to increase wire gauge size whenever possible. That way Voltage/Amps remain a constant. Gauges remain more accurate, battery is charged to it's capacity, everyting works, the engine starts faster(given variables)and the Alt is given a new lease on life.
Spent a lot of years messing about with DC when I owned boats, and sailed away . Cars are a lot easier to find grounds on than a fibreglass hull
. Learned a lot about DC electrical wiring after re-wiring a couple of boats for cruising.
FWIW