It's called Natsu no Shiori -epilogue-.
It's available on the a-nation '04 CD compilation. I'm looking for it right now.
http://www.avexnet.or.jp/avexdb/a-na...-17514_012.asx
Available here and at Amazon.co.jp.
http://www.cdjapan.co.jp/detailview.html?KEY=AVCD-17514
http://www.amazon.co.jp/gp/offer-lis...9688676&sr=1-1
It's going for a dollar or two on Amazon.co.jp~ You can buy it since it costs like 8 dollars only to ship overseas! :]