There is Sam's Silhouette Club up north in Knox that does Big Bore, as well as Riley Conservation Club in Terra Huate. As far as I know those are the only two ranges that participate in IHMSA Big Bore Silhouette. Those two ranges also do small bore silhouette along with Marion County Fish and Game.
You can also go to the IHMSA website but it is still a little early in the year to have the schedules posted yet.