I thought I had it bad trying to find 13.5s and XL socks for my kid... This is not profound advice, but online shopping is my go-to. I just checked out Zappos.com (owned by amazon), and they do show some athletic shoes, including a few styles of cleats, in sizes up to 18. (No half sizes above 14.5 though.)
Maybe find a basketball version of HSBaseball web and post a question to find out what parents of kids who are 6'8"-6'11" do?