Missing the plastic bag for shipping for my amp...

Yes those styrofoam end pieces can scuff up the finish but the suggestion of the Glad Wrap is your best bet. Manufacturers use bags because it is less time consuming and labor intensive.  If you're sending it back to manufacturer they will put it in a new bag as its faster for them. 

Roll of visqueen from a big box hardware store.  

ULINE sells plastic bags in several thicknesses, in a dizzying array of sizes.  No doubt, they have what you need, and may even supply the manufacturer 

Good news. I found the bag! It even has the serial number of the amp written on it 🙃

It was in my shoe closet... should I clean the bag throughly? I’m guessing there’s grit and bits of dirt inside the bag now...

I would turn it inside out, and just wipe it down with a damp cloth.Let it dry completely.  right side out, done.