I've installed a ZZP shift kit, ZZ Performance
Personally not a big fan. It has plastic spacers. It'* a OK kit for the price.
I've also installed a Intense kit in my own car, INTENSE-Racing.com: INTENSE™ Street/Strip Performance Shift Pak®
The price is much higher but has metal spacers and comes with springs and O rings. I'm very happy with mine.
W body store has one as well. W Body Store Performance Shift Improvement Kit
I have not used this kit so I have no idea how good it is. I also know of no member that has used it.
Or if your handy you can make your own. I have a write up in the how to area here and it has spacer lengths. You just need a brass tube from a hardware store and cut to length.