Sears Holdings Corp.’s deepening financial troubles have forced insurers and banks to raise the cost of guaranteeing payment to vendors, rattling the retailer’s supply chain as the company heads into the key holiday season. The move by these financial intermediaries — in an opaque but vital quarter of the retail business where makers of goods ranging from apparel to TVs seek to insure they get paid — comes in the wake of unusual steps Sears has taken to raise cash for operations.