If it's just a standard rubber plug that you need, pretty much any local marine store will have them. Most probably carry the Moeller brand and they are typically about $4 or $5 each. These should be very, VERY easy to find.
Check any well stocked hardware store (Ace, True Value). The section where they have those cardboard boxes of nuts & bolts, you'll find one with rubber plugs/stoppers. Grab a half dozen.
Just looked on eBay under Grady White parts and found the same plug for $5.95 plus $7.20 shipping, grand total of $13.15, and that's just for one. Wonder if I messed up and should have ordered those?? They are Grady specific with part # 11-124. :hmm