You can get the balljoints (GTi parts) from here:
www.japarts.co.uk or, most motor factors. As before, the part you'll be requesting is actually for a GTi but, it's exactly the same part.
It is definitely worth uprating the other bushes at the same time as, you'll need to take the wishbone off to replace the balljoint. It'll tighten up the front end a bit and, the poly bushes will last longer than the stock rubber ones.
I'd recommend you replace the wishbone rear mount with a Whiteline anti dive/lift kit as, this will help a little with your handling.
You can get the other bushes from here:
www.superflex.co.uk
The bushes here, are made in the same factory as the Whiteline bushes so, quality is good