No, onboard credits are not shown online anywhere. Sometimes, if you call and ask, you'll get an extremely helpful CM who might look it up for you, but it's not the norm.
You can purchase cruise credit via the "In-Room Gifts" link on the DCL website. Click on "In-Room Gifts" and then select the port you're embarking from, and click "Go". It's on the downloadable gifts order form mentioned at the top of the first page there.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!