In Stores Now, Now Available
Noun and の adjective
はつばいちゅう
年末ジャンボたからくじはまだ発売中ですか？
Is the Year-End Jumbo Draw Lottery still for sale?
これは、トーフグストアで発売中の手ぬぐいです。
This is a tenugui towel that's sold on the Tofugu store.
少年ジャンプ６月号、ただ今絶賛発売中。
The June issue of Shonen Jump is now on sale, to critical acclaim.
You have 発売, which means "for sale," and then you have 中 which is middle. An item that is in the middle of being for sale is now for sale, in stores now or in other words now available. You'll see this for items which have recently been released or when a shopkeeper wants to let you know that they're selling something now, finally, or whatever.
The reading comes from 発売 and 中 (kanji reading) put together.