I use this blog to get albums/singles of H!P groups, it's super helpful because theres so much on here. Unfortunately, not every group is on here (Coconuts Musume, Country Musume, Melon Kinenbi, etc. Also for some reason Tanpopo is hidden in the H!P albums page???)  and some singles and albums are just missing (W has one release on its page) But yeah, worth checking out MuSuMeTaNaKaMei if you dont feel like spending your money on apple music or cds. Let me know if you have any alternatives or places to get photobook scans/additional digitizations !

Peace!!