Personally I'd type "Mustang leaf springs" into the Craigslist search function and see what turns up.  When I need something local to save onerous shipping cost that's where I go.  Lots of good deals there. 

Another option might be to look at having the ones you have rebuilt.  If there's a spring shop in your area you may be surprised by what they can do.  They can replace broken leaves, change ride height, etc.  Look for shops that deal with heavy trucks; that's most of their business, but they are usually good people who will work with a project like this to help out a fellow car guy. 

That's a good point, be sure to check the reason why they are being sold.  In the past I've sold leaf springs that were installed, but has less than 20 miles on them.  The project ended up not being worthwhile and I parted the car out.  You'll also find people who bought them for a project and never even installed them.

The price you will pay for a pair of new, mideye, four leaf springs will be money well spent.  Save the coins on stuff that is easier to replace later, should it not work out.