Maybe your existing choir could spread themselves out at different Masses and deliberately sit in different places, and find a few new voices that way.
People often respond to being told they sing well.
Believe it or not, I have also had luck with the standard bulletin announcement. Try a little humor. Once I had the choir members complete a questionnaire in which they described what choir meant for them, how it affected their prayer and worship life, and such. I then used anonymous excerpts from these to create a flyer,distributed as a bulletin insert. I also use the bulletin to report success in recruiiting.