Design Within Reach has always been a great source for modern outdoor furniture. It can be an investment, but the impact of strong, unified furnishings does wonders for an outdoor space. I was pleased to see they have newly released a Walter Lamb collection, an iconic furniture line from the 1940's originally made from salvaged copper. In the right context, this collection could really be fantastic. Or if that doesn't suit and the early modern/midcentury feel is what you're after, there's always the classic Case Study House Terrace Collection and Bertoia.