Is there an aftermarket 6 speed manual transmission available for the Scion tC?

After market transmission changes are very rare and far from cheap. I would guess that the transmission change would in fact cost more than the amount you're going to save in gas mileage. And you would lose when you sell it because most ppl want cars in their original condition.

Not sure but the '09 Corolla XRS uses the same 2.4 liter engine as the TC and with a 6 speed manual from the factory. You might want to check that one out.

What engine/make/model/yr of vehichle are you going to be puting it in and where did the motor come from? This information would help a lot.

no there isnt but it may, i say again may be possible to fit a 6 speed from another toyota car, probably a celica gt-s.. but i said may be possible, youd have to do research check fittings and all that, because other things like suspension and mounts may have to be reposition, or modified to be able to sustain a little more weight. then there are other modifications to axel and drivetrain that may have to be done.... once again though maybe possible.

Lol, you sure give great information about your car for asking about such unimportant parts like a TRANSMISSION... besides, there are not "aftermarket" "mod" transmissions....