I'm not too sure about antique - as in heirloom kimono, but old kimono are readily available for cheap in Japan. Especially if you know where and how to look, which I'm assuming grandpa Gojo should. So I wouldn't worry too much about prices
@parkourse You use revenue stamps (収入印紙) to pay for certain documents, like visa extensions or passport issuing. You buy them at a convince store/bank/post office and put them on the form you hand in