Church choirs usually operate on a low budget. This budget usually has to cover music, stands, practice space upkeep, an accompanist, travel and a multitude of other things. You can get your church's choir something useful that they might not have the money to get themselves -- new hymnals, binders or music stands, for example. Another option is a gift certificate to a reputable music publisher, such as Schirmer. Talk to the conductor of the choir before you spend money on anything like this -- she'll let you know what the choir needs.
Church choirs members are often heavily involved with the church they sing for. Your church could get together and hold a potluck or dessert night to appreciate them. Bibles or other religious texts are never out of style, either -- but church members probably already have a lot of religious materials, so if you get something like this, try to make it unique or personalized. Church-related gifts could also be something small -- like a pin for each choir member.
If you're looking for a small gift, think about getting a commemorative gift. The most popular such gift is an engraved plaque. You could put the year, the occasion and a message of thanks or appreciation on it. You could also frame a photo of the choir performing. A choir can hang a gift like this in their rehearsal space and appreciate it every time they practice. It's also nice for new choir members to enjoy.
You may not have the money to buy a major gift for a church choir -- especially if you have to buy 60 of whatever the gift is. But showing your appreciation is the most important thing. A card or letter is easy and budget-friendly. Nothing raises morale at the beginning of a choir rehearsal like a heart-felt letter, read aloud by the conductor. Pick out a religious card, or make one yourself, and write down your feelings of gratitude and appreciation. That's all it takes.