As said, most likely the starter switch (ignition). It still could be an alternator failure, even though you were able to drive 300 mi.But, it's easy enough to rule out the alternator. First, the alternator could have partially failed say at 275 mi. and the battery was sufficiently charged to get you the next 25 mi.
Use a DMM and, engine running - lights on, check the DC voltage between the battery terminals - it should read 13.5-14.5 VDC, if other, your alternator is on it's way out. Alternators often fail progressively rather than all at once. It contains 3 sets of diodes which can fail singly changing the output voltage to the battery without a total failure. I'd do this simple check before ordering any other parts.
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