If you can't get something recommended by the manufacturer, you may be able to find something suitable by taking measurements or reading the size off the existing seat post. Sheldon brown has a database of seatpost sizes, but sadly it doesn't seem to include Viking or Hopper.
I guess there are three things you should look for:
- Tube diammeter: Must either match your existing post or you can sometimes use a plastic shim to make up the difference.
- Layback/Setback: How much the bracket at the top of the post is offset behind the tube. Larger numbers put your saddle a bit further from the handlebars.
- Strength: I'm guessing really long seat posts for folders have to be pretty strong compared to those on standard bikes, so make sure it is specific to a folder and up to the job.
You may be best to take the old one into your local bike shop and ask if they can find something for you.