"This item is not available at our store yet."がよりナチュラルな表現です。
入荷日はまだ未定です。
"We have not yet confirmed when the item will be available at our store."
"The date for item's availability has not been confirmed yet."
〇〇に入荷予定です。
"We will have the item in stock from 〇〇"
"The item will be available from 〇〇"