I don't trust anyone to pack them but myself. There are a few places around that sell boxes. Uhaul has some good t ply boxes, and some of the better packing stores do too (not the UPS store). Buy Styrofoam insulation at hope depot for padding and cut it to size. If the speakers are big and heavy I would build a crate, or at least reinforce the box using 1/4" plywood.
