Summit Racing, CJPony Parts, Latemodel Restoration.....take your pick. Every Mustang Performance Shop in North America sells them. They range from about $60-$360 depending how extreme you want to get.
I've got a set of Maximum Motorsports Full length connectors in mine and they're top notch. I also made the set in my previous Mustang and IMO, they were every bit as good. Make sure they're full length and cross braced and you can't go wrong.