How does the black part come off ? - In bits. I have found they get quite brittle and cracked.
Where to get them? Here -
http://cobsmorristuff.co.nz/index.php?route=common/home
I can't see them on the web site but this is definitely where mine came from. They have only recently been manufactured so may be a bit new to get on the website yet. An email would be a good idea, to the address shown on the site. (I suppose I should point out that this isn't me and I'm not advertising. I'm just a customer.)
A bit of additional info - The unit is sold as one complete piece. The green end is glued on and not screwed, but with an LED it should not need to be removed. The plastic it is made of is meant to be UV stable. I have found the light to be well visible even in bright days.
To fit, remove the old one (I made a cut down the length of the old black bit and 'sprung' it off) and the wire(s) in the stalk. The old wire(s) will need to be unsoldered. Then it is just a case of inserting the new wires into the stalk and pushing the new end onto the end of the stalk. It grips firmly with no glue. The new wires need to be connected to where the old ones were removed from, being careful of the polarity.
I've still got the old bits so I can get some photos of the process if needed. Now, if I can just find that box that I left in a safe place in the garage.