It's a Futaba S3801 heavy duty servo. Goes for about $55 USD. Available at:
194 oz/in torque at 6 volts, which is what I run it at.
BTW, you ought to be able to just use a 'Y' connector for your two servo setup and flip one servo backwards, shouldn't you? Unless they have to be in a special position.
When you finish the slider, I'd like a look. My rig is a little far away from the driver and could be improved by mounting it on a 'sled' to move forward after flip up.