Are there any prepaid travel cards (like the Amex GlobalTravel) available in Japanese Yen?
Travelex offers cash passports:
You can choose to pick up your currency in one of our many stores or have it delivered directly to your door.
It covers many of the major currencies, including Japanese YEN.
You can buy at their counters in most major international airports, or online in advance.
Japan is very much like a cash society. Hotel and travel bookings, as well as big-ticket items, should be purchased on your credit card and if possible make the most of your card's interest-free days feature.
You need to be wary of travel cards, compare the exchange rate on offer from your travel card issuer.
As others have stated Japan is a cash society, and many places do not accept credit cards. There are cash exchange locations at the airports that are operated by banks which offer better rates than Travelex. I've have had good luck getting cash from post office ATMs, and 7-11 stores.
For convenience I do recommend getting an IC card. SUICA and PASMO are the 2 major ones, and they can be recharged at most train/subway stations. The money on the card can be used for travel, vending machines, some lockers, and many stores. It keep you from having a pocket full of change. Super handy.