Sonnc Bloom host a range if Ableton Live packs, presets and courses. They've just released a free Mellotron SB Choir sample set, which features a timeless choral sound from the vintage instrument. Here's what they have to say:
The Live Pack contains the individually sampled Mellotron 8 Choir in one Simpler preset as well as four fully macro mapped Instrument Racks, all including reverb and one of the various delays in Live . The Live Pack is self-installing and made for Ableton Live 9.7 Standard and up.
The sounds were sampled by Taijiguy and a number of freebies are available from their site here: https://sonicbloom.net/en/category/freebies/