Depends on how many you want to ship, really.
If it's just a few, use a box about 8 inches square and line it with foam. Make sure you double bag and pack so they can't shift around. Use air permeable bags.
Check with your LFS, they may have a foam shipping container they could give you and you can cut it up to line the box with. Talk to them about how they receive shipments.
If you have lots of fish, use the shipping box as is, but put it in a cardboard box.
You'll need to ship via air mail, or air freight. The airlines will pick up at your door and deliver, not cheep though.
By the look of your signature, you are in B.C. There are several members on Canadian Aquarium Connection who ship regularly. Join there and ask Melody or Blaine.
Good luck.
Rick