I looked this up in hemmings motor news strictly sticks in N.j. richie 973-786-7500 or theirs this guy in NY larry 631-351-4837 they both say they stock parts for muncie good luck on finding what you need hope this helps
You can still probably get the gears at a Chevy dealer.
TCI Transmissions has a chart and a calculator on their web site that will help you pick the right plastic gear. I think they sell the gears too.
The Muncie transmission uses the same plastic gears as a TH350 or aluminum Powerglide.