If this happens at our LQS they tell us up front. then offer to let us pick a different fabric to finish. This did happen to last years "buck a block" BOM. The border fabric was unavailable for probably half or better of the people signed up, but they let us pick something else to use. I would not have been happy if they had just substituted something without telling me. Speak up and let them know that you would have appreciated being told up front.