I bought my TCAs from Compbrake, as the quality is good and their prices are the best. They were very easy to fit, and as John says, make setting the Camber so much easier.
Compression struts would then make setting Castor just as easy. Again as John says trying to set the wheel geometry up using top link shims as Marlin suggest is not easy at all. Each shim fitted changes both Castor and camber, so there is quite a lot of trial and error to get to the desired Castor and Camber at the same time. With adjustable TCAs and Compression struts this would be so much easier, allowing it to be done at a professional Wheel Alignment outfit.
It is also important to set up the rear suspension geometry, as many Cabrios appear to have an in-built Marlin manufacturing alignment fault.
Toe-in is often excessive on one of the rear wheels.
The solution is to fit shims between the rear hub carrier and the trailing radius arm. It is again trial and error, but worth the effort.
Peter Licence posted some good advice on the MOC