You can tell if your car has got a lsd pretty easily by spinning up the rear end. Both wheels spin and the tail goes out in a nice controllable fashion. None of that tankslapping, fishtailing oh er missus that comes with one wheel spinning then gripping and spinning again.
For an English axle, which mk2 have, TranX, do a good one. A new one will cost about £400 and you'll need bearings gaskets ect to fit which will cost a max of £60.
To tell if your axle is fitted without doing doughnuts, simply jack the rear end off the ground with the handbrake off. Turn one wheel manually. If the opposite wheel goes in the same direction it's a lsd, if it goes the other way its a standard diff.
Looking at them on the bench a standard diff will have a nest of cogs where as an lsd wont have any showing just a solid set of plates. Check ebay out for pictures.