I had someone show me this insert a while back. I was looking for one and I can't find it anywhere.

Description: It's a piece of plastic that is shaped like a thumb that you would glue into your thumb hole. Reason for it. It basically forced your thumb to be straight. You could not knuckle the ball with this insert in the ball. It accounted for the curve of your thumb at the tip.

Anyone know if they still make these things?

You mean this:  http://www.buddiesproshop.com/p-1956-ebonite-thumbs-up-thumb-straight.aspx  ?

This is basically what the TIERED TAPE technique by Joe Slowinski does for your thumb, and I love it.  If the Thumb Straight works the same, it's much easier than cutting up pieces of tape, but doing it with the tape is more customized for your own thumb. 

I just noticed my teammate doing this last week. The only problem with this, if you don't prep an extra set for leagues, it's a b!tch to work on it during leagues.

Juggernaut,

Yep! That's it! Thanks.

The problem I have with tier taping is you don't need to tier down. Its that last 1/2" of your thumb that needs the space filled.

When I construct my tiered tape, I make several at the same time.  Right now, I have 3 sets in my tape box, plus I use them in a switch grip. 

The tape would be constructed to fit your thumb, so it is totally customizable.  If you only need the last 1/2" of your thumb, you can make it that way.  Mine is really fat on the tip, like 12 pieces high, but it tiers down to only 2 at the bottom.
Mine goes 12/6/4/2. 

As far as the insert goes, does anyone know what it is made of?  Is it customizable because every thumb is a little different?  Also, how does it go it and stay in? 

The Thumb Straight will give you the feel of  a slight  thumb hole pitch change as well as lengthen your span slightly, due to the thin strip of rubber that comes up the grip side of the thumb in the thumb hole. I tried the Thumb Straight and hated it, gave them all  to a friend of mine and he likes them.