Personally I would go to the hardware sellin' emporium and get some stainless steel bolts in the appropriate size, then install them with some anti-sleaze to keep everything easily maintainable. Be aware though that you should install them with a torque wrench, they're stronger than Grade 2 but weaker than Grade 5, which is probably what the stock bolts are (or 8.8 for metric which is roughly equivalent.)
If you anticipate serious rock crawling and shear loads on your diff cover, get Grade 5 or 8.8, but they'll be zinc plated and should be painted over after installation if you're serious about details.