after I read the thread about gags here, I want to give you a hint how to create your own ballgag, which is reasonably safe and made of readily avaiable materials.
First- the ball.
Someone suggested to take any rubber ball of the apropriate size- this is wrong. Many rubberballs contain chemicals which have unpleasant side effects if they get solved with your mouth's salvia.
What you need is a rubber ball for babies- I don't know the correct english word, but a one-to-one translation from the german word would be "biting ball". They are *MADE* so babies can bite on them, testing their teeth.
As for size, I would suggest 40-50mm, but tastes varies.
Second, the "string".
This is used to hold the ball in place. This can be either a simple string, which you will knot behind your neck, or a small leather strap with buckle.
Now, how to make this thing....
Somehow you will need a hole in the ball to get the string through it. However, you can't just take a driller and drill a hole into the soft rubber, right?
Well, yes and no. Yes, it doesn't work. But there is a trick
Put the rubberball into a freezer. Give it a day or so, so it's frozen through and through.
*NOW* you can drill a hole through the ball without any problems.
The rest is easy...simply pull the string through the hole, and voila, ready is your selfmade ballgag (just keep in mind not to use is as long as it's frozen

Drop me a line if this was helpfull to you, or if you have more advice.
Cheers
FF