I don't know that it is critical to replace. I bought an 03 that Mickey Mouse had made a winter home out of mine. I just took a vac to clean it all up and replaced the cabin filter. It's works just fine.
If you were looking to find something from VW; you probably would have to just replace the whole part (insulation not available separately). I'm sure you could find some suitable material (foam, dynamat etc.); install it with a adhesive and it would be fine. Insulation, foam etc.; could probably be found at a home improvement store.
Probably Dynamat would be overkill but you could install it other places to deaden sound as well.
This company; seems to sell some of the cotton style that looks very similar to the stock insulation ("Acoustic Cotton"; of course seeing how yours/ours rots away you might want to step up to the more modern type like dynamat. Check it out here: