When I was organizing events, one municipality-owned site required $1M of "PL PD," which is personal liability and property damage. Cities and other governments have deep pockets; they want to be names as "additional insured" on the certificate. Seems like the coverage was $40/year at the time.
CDSS used to offer caller insurance, membership required, and coverage was extra (through Callerlab). Turn-time was not speedy. A couple of years ago the local dance society found that the National Folk Assn coverage met venue requirements at a much lower price.
To find an insurer, just do a search on "special event insurance" and
your city/state or province. Your local or personal agent can write the
policy. There are underwriters for all kinds of activities and events
(bungie jumps, beer festivals, Civil War re-enactments) and an open,
public contra dance is well within these confines. The price is easily covered by a few people's admission fees.
HTH
--Karen D.
in the grand and glorious state of Michigan