You mean the rear anti-roll bar ?
The brackets are already there on the chassis, you'll need:
Anti roll bar (i used a 325i bar)
2 bushes (I used standard items)
2 mounting brackets with bolts
2 drop links
2 trailing arm/droplink brackets with bolts.
I got all the bits from a scrappy for Ԛ£10 and fitted new bushes and drop links at a cost of Ԛ£40.
It is a bit of a squeeze to get the bar in but the results are well worth it
Cheers,
Iain T