Welcome to the forum.
The ST is a Thai build and the chassis at the front is quite different to the Spanish-built STX (my STX's VIN starts with VSK, indicating Spain - the ST should start with M).
There's a few places putting gear out for the ST, give 'em a call.
Are you going to do any outback touring? If so, I'd suggest going a steel bar, because alloy really won't fend off too many roos at highway speed. If you just want to hold up some driving lights, go the alloy - it's much lighter. The steel bars are heavy enough to warrant a second look at the front springs - something that Nissan are doing with the recall in the '09 STXs that have enough weight on the front (bar + winch).