Start with local used bookstores. Many have discount bins or shelves. Other great sources include charity shops - there are chains, such as Value Village/Village des valeurs, Renaissance, Goodwill, Salvation Army shops, etc… You can also try Church bazaars and rummage sales. Local libraries often have a yearly book sale, and some even have a sale table for books they’re phasing out, or for which initial interest has died down, so they no longer need as many copies. Finally, if you’re lucky, your area might have a giving box or free library, where people leave objects they no longer want, but that are still in good shape, for people to take, we have found some great books there, and have also left some for others.