Five volumes, by Ikushima Miya, published in Japan by Houbunsha. The main pairing is Hijikata x Okita, but the stories eventually give all of the major players their own relationships. Unlicensed outside of Japan, and out-of-print, but not hard to find. Please visit our downloads section for the first four volumes, translated long ago by Hana-mi.net.
Soshite Haru no Tsuki ISBN 9784832280670
Akeyuku Sora ISBN 9784832280946
Yakusoku no Oka ISBN 9784832282339
Chiranai Hana ISBN 4832282018
Ten to Chi to Kaze to ISBN 9784832284227