They are fluid filled bushes, and officially non replaceable. However they certainly are replaceable as I did the ones in the bottom of my Bilstein socks on my TF (same bushes I believe). I got mine from Bilstein after chatting to a very helpful guy there, but I think the eBay item below is the same thing:
Item number 351542059423
Best to double check but I'm pretty sure they are the same thing! You need to press the old ones out, then press the new ones in. I used a cheap £50 press from eBay... Used it since for other stuff so it's been very worthwhile.