RJ
I'd like to know the answer to that, too! Most of my sheet music was inherited from my mother, who was a gifted pianist, also have some from my similarly musical nephew (her grandson). I'm a bog-standard pianist and sing in choirs, too, so also have lots of choral music. I've even got my grandmother's Tonic Sol-Fa version of the Messiah! She couldn't read music and Grandfather thought this would be easy for her.
My great-grandfather used to own a music shop, selling, repairing and tuning musical instruments, but he also sold sheet music. My grandfather, his son, had the job as a lad of going to the railway station every morning to pick up parcels of sheet music, which arrived every day from London, according to the shop's billhead.
I have sometimes seen sheet music on sale in charity shops, so you could try that way if you have any charity shops near you.