I know this is not what you are looking for but perhaps you could set up something along these lines, rather than try to source karaoke material? - I have seen posters advertising Dementia singing groups - there is an organisation which co-ordinates these in local areas, can't remember details but I saw a poster in Oban, think there is a group there -- a leader (and helpers?) organise these, it is very beneficial to Dementia sufferers who can in fact remember songs from 'way back and thoroughly enjoy the opportunity to sing in group.

Back in the 1950s and early '60s group/ choral singing was very popular, I have picked up many songbooks in charity shops (there's another thread on this) which were produced with a broad cross section of songs in them - many were folk songs. So - song books / song sheets might be another option?