I agree, reusing the OEM bolts should be fine with a little blue Loctite. I've reused suspension fasteners many times and have never found signs of the bolts stretching. However, I don't drive my Mustang in the winter, so I don't know how much the corrosion protection is compromised. As Norm said, TTY bolts typically have a torque and then an angle spec for tightening them. Head bolts and rod bolts are common examples.
If you know the thread size, pitch, and bolt length you need, you can also buy high-strength fasteners from McMaster (or similar). However, this really only makes sense if you need a lot of them because the shipping cost will probably negate the savings over OEM fasteners. If you have a local Fastenal store, that's another option that will save you the shipping expense. I would also advise against using "All-Purpose" fasteners because they're typically low grade. They're fine for low-stress applications like wiring harness brackets, but I wouldn't use them for critical suspension components. Also, do not use stainless-steel nuts and bolts for anything that requires a lot of torque! They might not corrode and offer a lot of strength, but without anti-seize or something similar, they will... seize. Once this happens, they're a PITA to remove. It's fine for small stuff like M6 and below, but use something like zinc-plated steel for anything larger.
Back to the original question - if the vendor's replacement bolts are grade 10.9 and are the same pitch and diameter as the OEM bolts, I'd just use the OEM torque spec. If the replacement bolts have coarser threads... maybe just reuse the OEM bolts if the vendor still isn't helpful.
