Thieving Scoundrell!!!
Lever is from an MGB. The MGA is very similar except it has a push to lock button rather than a push to release.
I got mine from an MG specalist in Richmond that I cant remeber the name of. Cost me about $150 from memory.
I went for the painted one and sand blasted it and gave it a lick of paint. They have a fairly nasty plastic grip and button so replaced them with a machined up aluminium one that adds some bling and feels nicer when pressing the button.
Cheers