I don't know why you need one, but beware of used regulators. Especially the driver's side. The clutch that holds the window up is a wear item and too often doesn't work or at least the shaft is very wobbly. It's this clutch that brings many to purchase a new one.
I have posted before on these clutches and how they can be replaced or overhauled back to life. If that's your problem, do a search. You can put a good clutch from a passenger side regulator into a driver's side regulator.
Like I've said before, I think the aftermarket should provide replacement clutches instead of making us buy entire regulator assemblies. I've heard the repop regulators aren't as robust as the originals. Plus, it should be much cheaper to replace a clutch than an entire regulator.
Formerly known as Bashed68.
Resto-modded '68 convertible.