Search for Mustang steering shaft, that should pull up many links for buying the aftermarket options, from multiple brands.
Flaming River is one top brand,
Borgeson is another, and
Maximum Motorsports also makes a select few. I got one for my Mark VII made by
Borgeson IIRC, which was made for a 94-04 Mustang, it was more than most, about $320 and had a nice NVH(feature like a rag joint) built into it for street use. Most of those shafts are made with solid precise u-joints, larger than stock and strong, but again they don't come with any boots to protect the joints long term. Those could also be covered as others have suggested for the stock shafts, a generic bellows or a tire tube cut and zip tied over the joints.
Here are a couple of links that can give an idea of what aftermarket shafts are out there.
This is an Explorer shaft by Dorman, $257 with a good looking boot over the joint;
Free Shipping - Dorman 425-368 with qualifying orders of $109. Shop Steering Shafts at Summit Racing.
www.summitracing.com
This is a search result on Summit for Mustang shafts;
Find FORD MUSTANG Steering Shafts and get Free Shipping on Orders Over $109 at Summit Racing!
www.summitracing.com
This is one of those results, a universal shaft by Borgeson which includes boots over the slip joints, for $357 though;
Free Shipping - Borgeson Universal Telescoping Steering Shafts with qualifying orders of $109. Shop Steering Shafts at Summit Racing.
www.summitracing.com