Pursonic Replacement Toothbrush Heads

What is this?

Replacement toothbrush heads for my Pursonic S520.

Why did I buy it?

Unless you buy toothbrushes to collect them, eventually you will need to replace the toothbrush heads. 

These wound up costing about $3.50 each, and don't seem to be readily available. I don't know if I would buy a Pursonic if I had to do it again.