A trash pump may do it, but I'm wondering how much water you'd pump out of the pond at the same time. I have no idea what those bags are called, but my guess is that they're pretty expensive.
O.K. after a quick google search, they are called dewatering bags. Here's a place that sells them. It looks like they are a one-time use thing, slit open the bag to get the muck out.
dewatering bags It looks like you'll have to call them for a price.
