if you don't have the CD album, you can buy the official instrumental version from mora (Sony japanese drm-free music store, there are tutorials on the web how to buy)
ロミオとシンデレラ (Instrumental)
moraは高音質の音楽ダウンロード・音楽配信サイト(月額会費無料)。ミュージックビデオ、ハイレゾ音源も充実。多彩な決済方法でPC、iPhone、Android等から簡単購入。購入した楽曲はいろいろな端末で10回まで再ダウンロード可能。
mora.jp
and for ワールドイズマイン , there isn't mora music store page, so you should search for the official album CD for example on yahoo jp auctions with a proxy shopping service, I have the album CD and I confirm the second track is the instrumental version. Since is very rare album, on yahoo auctions is possible to find it, but at high price (more than 10.000-20.000yen usually).
you can search some videos of the instrumental or karaoke (カラオケ) version, or also some japanese covers and check in the video description if there is the source: