I have a 3/8" bit screwdriver but the bolt I need to unscrew is even bigger than that and TIGHT, so using that screwdriver is starting to strip the screw a bit. I can't find a bigger one anywhere online.
Here's one from Grainger.
Sounds like you might need to use some frozen-bolt techniques, too (penetrating oil, carefully applied heat, etc.).
This sounds like a job for a socket wrench.
Because a socket wrench has a long handle, it acts like a lever and you can get a huge amount of torque from it, MUCH more than you can get from a screwdriver.
You can get driver bits for those as well, but since you said 'bolt' and it's 1/2", that it will have a hex head on it that you can use a socket bit on.
Using some penetrating oil or WD-40 on the threads may also help to loosen it if it's rusted or corroded.