Anderson Flats are awesome. They are very very sturdy and seem to hold up very well in the sun. They are definitely of stronger grade than pond baskets. One thing I've noticed is that yamadori collectors will cut them in half and attach a thin wooden board on the cut side to pot smaller collected material.
I bought mine from the same vendor linked by Ray. I found that there were a number of bonsai sellers that charged considerable markups for the flats, so like most things, it's cheaper when you can cut the middleman out.
Don from Gregory Beach Bonsai also sells some heavy duty plastic squat containers of various sizes. You might want to check with him too. I haven't used the ones he is selling, but they look pretty sturdy.