The thick black cable/cord does not come pre-sold in a package. It would normally have to come off of one of those spools that a store employee would cut to length for you. HD was about to close, and I happened to notice a rack with various types of cords that must have been over-cut pieces or un-needed returns. There happened to be a 12/3 Romex cord of 15' in length which was calling my name.
My rule of shopping at HD and Lowe's is to do it like you're buying PVC plumbing for a job: Always buy more than you think you'll need! What you don't use, you can simply return on your next visit for the next project and get store credit, or a refund if within 30 days.
I actually just stopped by HD last night to see if they got in the 30amp versions of the Twist-Lock wall socket/receptacle and Plug, and they did. They also had the "Female Plug" end for a cord so I could make a my adapter for being able to plug into a NEMA-1450 outlet.
But then I got home, and checked my own drier outlet receptacle......and its a NEMA-1430 socket. LOL Now I'll have to make another adapter!
However, I experienced sticker-shock when I got to the checkout lane. The Twist-Lock 250v 30amp female socket/plug to go on a cable, was $39! The male end and wall receptacle units were at least $20 each. However, now I can go get a refund for the 20amp Twist-Lock versions since I saved the packaging and receipt.