I think the only way you can find one is if your lucky enough to find someone willing to part out their transmission or buy a complete used T56 with one in it or wait until tremec makes them sometime this year hopefully they are on back order. Best bet just take it easy with a 10 spline until their available, if you find any 26 splines ill take on forsure.
I found a guy who had brand new tremec one in December. Had to pay 750 USD. With shipping and exchange was around 860 CND.
Texas Divetrain Performance cuts gm ones down to fit. They have them usually and there around 450. Some guys on here have bought them and they've worked out fine. Just a suggestion. Good Luck
Did you get one yet? Cabbage was right Deitick motor sports in texas "spelled it wrong" has em I just ordered mine today he's got one left comes with bearing pressed on and something else 479 ouch I hope it will be good insurance if i ever get er runnin