While the specification allows for ½” A490 bolts, they are so uncommon that they are not mass-produced. This means they cannot be found on the shelf and would have to be custom-manufactured.
A workaround besides just moving to a larger diameter would be to switch to J429-8 because they are easily found in a ½” diameter and have almost the same chemical and mechanical properties. The only issue to consider would be that J429-8 bolts are manufactured with a standard hex head and A490 bolts are manufactured with a heavy hex head.
Of course, any substitutions should be approved by the engineer of record.