Local schools, universities, local choral society or chorus (their conductors may have contacts?), even your nearest cathedral or larger parish church (depending where you are) may have more information. There are also various companies that hire out professional choirs but they do get quite pricey! If you only want them for the hymns though, I would probably stick with your church choir anyway - all that's needed is a few confident singers to bolster your congregation! However, if you want them to sing something by themselves then you might need to get in help...
Whereabouts are you based? Might be able to be more specific with where to approach - church music is my trade. xx
