From your window(s)*! Check this out (a few years old, but recently thought about it and thought I'd share):
http://www.ted.com/talks/britta_riley_a_garden_in_my_apartment.html
Their website is here (check their d-i-y and our. section):
I'd also like to add, have some flowers around for the bees that are so busy pollinating our greens.
*No, I don't mean the operating system.