Amazon Japan has the Shigatsu wa Kimi no Uso soundtrack available for download: "Shigatsu wa Kimi no Uso ORIGINAL SONG & SOUNDTRACK". It's an MP3 download (so not lossless, unfortunately). The whole OST is 3000 JPY (~25 USD), or you can buy individual songs (but this quickly becomes more expensive than just buying the album).
The OST does not appear to be available (losslessly) on any of the sites linked in mivilar's comment, but the Ongakuchou (consisting of the classical pieces from the show) is, at mora: "Shigatsu wa Kimi no Uso - Boku to Kimi no Ongakuchou". I've never used mora, but it appears they have a FLAC version of the album available for 3200 JPY, as well as a 320kbps AAC version available for 2100 JPY.
Amazon Japan has a setting somewhere for translating the UI (but not product descriptions and so forth) into English, but I'm not seeing anything similar on mora, so you may want to find a buddy who knows Japanese and/or bumble your way through Google Translate.