CRM 2013 has gone metro-style and so should your icons for custom buttons and entities!
icons8 (http://www.icons8.com) has a large (1500+) metro-style icons perfect for this. They are free for personal and commercial, use, as long as you include an attribution and a link back to their site. Alternatively, you can purchase them at a very nominal fee of USD 149.
They are currently available at various sizes – 26×26, 32×32, 48×48, 64×64, 128×128, 256×256, and 512×512. Size 16×16 isn’t currently available, but re-sizing is permitted, even for the free version, and you can do so by using a batch image tool.