If you're flying anytime soon you can always try a currency exchange booth. I've gotten around 35 coins this way on various trips
I know it's more fun to use 100 jpy coins, but fwiw I've successfully re-calibrated both 100 and 50 yen coin mechs to take nickels. 50y coin mechs are easy, usually just the one screw for the thickness of the coin and you're good to go.
my shmup cab uses 100 yen, my other 2 cabs just use nickels. Plus they'll take 50yen too without any adjustments.
I have 25 coming in next week. Call your local coin shops and you will be surprised what you might find. For the oregon people, Justin @ Liberty coin and currency's beaverton store is who hooks me up!
Every bank and exchange I called only did paper money. Most won't do coins because of shipping weight. Better make sure that Yen is actually coin and not paper money.
Did you just order a current 100 yen coin? When i go on ebay there are tons of variations. Is there anything specific about them? I had read there was a larger size coin too.