Where can I find one of those football on a string things that helps with your throwing?

You are in luck! I happen to live with a man who loves all things football! The trainer you are speaking of is awesome! It is called the RZ Trainer. It will help you improve your game in the Red Zone. Practice passing and catching by yourself with this new 2-in-1 football trainer. Patented design allows the ball to return to you in one bounce. Learn to throw the perfect spiral and improve your "soft hands" catching. Improve fumble recovery skills or passing at targets. This is a Quarterbacks dream training system! It can be found at touchdownskills for $39.99! You get all these features with it:

• Ball has a tacky surface and is made from rugged composite leather
• Pro-grip lacing for a "real game" feel
• Heavy duty elastic cord is specifically designed for a "one bounce" return (TM)
• Removable Youth-sized ball - use as a trainer or a football Adjustable Neoprene Belt Fits Waists from 22" - 38"
• Best suited for ages 9-14 but can be used by all ages

The Drills
Mobility
The RZ Trainer is a completely mobile trainer. Take advantage of this by hitting targets on the run. Practice throwing after completing backdrop steps or while on a roll out.
Catching
Throw the RZTrainer in different arcs to get a variety of different returns. Including this in your drill with improve your catching ability and help develop "soft hands".
Fumble Recovery
By throwing the RZ Trainer underhanded, the ball will come back in a tumble, allowing you to practice fumble recovery and improve your hand-eye coordination.
Passing Accuracy
Improve your passing accuracy by using targets. Use cones or natural markers such as trees or poles. Throw towards target to improve passing accuracy.
I added more shopping choices below to help your game but not damage your wallet. Enjoy!