Avoid aluminum screws at all costs. Alumimum and ferrous alloys hate each other. At noted, stainless steel screws will be best for not rusting. Brass isn't bad. Don't forget that if you don't need a lot of holding power, nylon screws are great, but may not withstand UV radiation from the sun so will deteriorate. Farm supply stores are often a good source for oddball hardware. Our local one has both SAE and metric sizes and various materials as well. They also have larger bolts graded for higher strength.