I have attended quilting retreats at a 4H camp owned by neighboring counties, and also at a Bible Camp. The facilities were excellent, planning was too. Seems there is always a 4H member who is known for her cooking skills and she is the designated cook. Each day, there is a duty roster for those who help with prep and clean-up. The biggest groaner was that we were expected to clean for the next group before we left. When we are packing to go, cleaning toilets and vacuming the floors are not what we want to do. The 4H camp had plenty of tables and folding chairs. Most sewers can bring their own folding chairs and tables if alerted. Several of the women even brought their own upholstered easy chairs. Rolling desk chairs can be broken down to two pieces.
I would not think a library or church would be adequate when you need a place to sleep and to leave your supplies out without fear of losing them. I have heard of a few small quilt stores that have upstairs space for retreats. Some of the teen 4H groups have set terms for cleaning after a gathering. This is one way they fund some of their projects.
There is a retreat center closer to where I live, but their classes are far too expensive for most of us, even though some of the quilt shops near the I-5 corridor in SW Washington use it and often feature a well known instructor.