I picked up a complete original used set for like $40 shipped a year or so ago. Out of the bunch, 5 or 6 were very good and only one was really kind of iffy. It's always nice to have spares around, too. Hit up a few parting out threads on the classifieds and you'll probably be able to find something.
Here's the difference between the new OEM lifters on the left and the original OEM lifters on the right. They are both made by INA, but the newer less reliable ones spell INA in large letters whereas the originals have a very small INA logo and another triangular looking logo found on other Porsche parts.