You might as well just replace the whole panel
There's absolutely no sense trying to hunt down the correct grounding bar for your panel -- it's a FPE Stab-Lok, whose breakers and busbars are both dangerously defective. As a result, you're far better off simply getting a new panel -- you could even go with a 12 or 16 space, 100A, outdoor rated (NEMA 3R) main breaker panel if you're that pinched for budget, although I'd recommend a 24 or 30 space panel instead.
If this is directly off your meter, you'll need to coordinate shutting the power off with your utility, by the way. (Many utilities can do this remotely for free during business hours, even, thanks to smart metering!)