They're available from quite a few places, depending on what you're looking for. Summit racing sells both the XHD and Super Stock springs made by Mopar Performance, as does Mancini Racing and Jegs. Firm Feel also sells leaf springs, as does Hotchkiss. Hotchkiss' springs also provide a corrected geometry for better handling, but they cost almost twice as much. I believe Summit also sells generic leaf springs in addition to the Mopar Performance pieces. ESPO is another place that makes springs for the e-bodies, they have quite a few options as far as ride height (+1,+2, 0, -1 etc), but they are also a little pricier than Mopar.
I will say that the XHD springs from mopar ride well, but if you're looking for that muscle car stance with a decent rake you won't be very happy with them, as they sag quite a bit after they're broken in. This does not effect ride quality at all, but you will keep the factory stance with a pretty low ride height in the back.
If you're looking at keeping the stock stance, and don't need the springs immediately, I have a set of XHD's on my car now that will be replaced with super stocks in the near future that I'd be willing to sell. I'd guess they have no more than 7000 miles on them, as I installed them last year. I'm replacing them with super stocks as part of a rear end change and 1" spring relocation to fit some bigger tires in the car, not because I'm unhappy with them. They're on the car in my signature picture, but keep in mind the front of my car is lowered by at least an inch, if not more, from stock.
