For anyone interested, I replaced an alternator due to an oil leak from the distributor on my 1995 SC300. I searched this site for the distributor o-ring size or part number, none found. A few members asked the same question but no replies.
Purchased from a Toyota dealer, part number is 90099-14109 sells for ~$2. AKA M3.5 X 23 which has an ID=23mm X OD=30mm X width=3.5mm.