cheapest place i found it:
http://www.vwspares.co.uk/product_info.php?products_id=1450if it was me i would find mk3 calipers, rebuild with new vag seals in the comfort of the garage then wait for a warm day next year and swap over with new handbrake cables. which by coincidence is exactly what I did a few years back

as i say work the handbrake lever and watch the piston. if it doesn't move the mech is fecked internally and it needs replacing. obviously if lever is stiff or seized that's also a clue that Bad Things have occurred inside the caliper. with the cables i set them so there is a little slack when the lever is fully off, if you have run out of adjustment then that suggests the cables have stretched and are in need of replacement.
if you undo the screw at the back of the lever surround, pop it backwards then lift off you'll see the cables and the nuts. this is an easy job which can be done in the relative warmth of the car, so start there before you do anything else
