You don't need to buy anything to take care of the problem. Just do this:

• Start with your guitar slung in place with the strap, just as you wear it to play.
• Pass the jack cord from the back of the guitar to the front, between the guitar body and the strap, just above the endpin where the strap is attached.
• pull through enough slack to allow you to plug in to the jack.
• Pull back just enough of the cable slack to ensure that its out of the way, but not applying any stress on the cable connector.
• You're good to go!

While I think of it: If you wear a belt, get into the habit of wearing it with the buckle positioned off to your fret-hand side instead of in the middle. No more buckle-rash on the back of your axe.

You're welcome. :)