Exploring the utilization of white spaces (vacant VHF and UHF TV channels) is a promising way to satisfy the rapidly growing radio frequency (RF) demand. Although a few white space exploration methods have been proposed in the past few years, they mainly focused on outdoor scenarios. In this paper, we propose a novel cost-efficient indoor white space exploration method by exploiting the location dependence and channel dependence of TV spectrum in indoor environments. We first measure the UHF TV channels in a building, and study the spatial and spectral features of indoor white spaces. Then, we design a cost-eFficient Indoor White space EXploration (FIWEX) mechanism based on the extracted features. Furthermore, we build a prototype of FIWEX and extensively evaluate its performance in real-world environments. The evaluation results show that FIWEX can identify 30.0% more indoor white spaces with 51.2% less false alarms compared with the best known existing solution.