Make Your Own
Or you can make your own tie downs by digging a hole in the ground under each corner of your shed, dropping a metal strap or rod into it and filling the hole with concrete. Then attach this anchor to your shed floor. Either of these methods will provide some protection against your shed blowing or washing away.
Usually you will know ahead of time if these will be necessary for your new wood shed. History in your area will tell you what to expect. But most of the time building or zoning codes will require these tie downs if the hazard is frequent in your area.