Many service providers distribute various kinds of content over the Internet. They often use replica servers to provide stable service. To position them appropriately, service providers must predict the demands for their services and provide computing capacity sufficient for servicing the demands. Unfortunately, predicting demands is difficult because demand for a service usually fluctuates. Our research group is developing ExaPeer, an infrastructure that apportions computing capacity to services running on hundreds or thousands of trusted machines all over the Internet. In this paper, we describe ExaPeer's approach to dynamically selecting candidate spots for replica servers. The runtime system in ExaPeer detects fluctuations in demand and then dynamically selects candidate spots on which replica servers should be placed to best meet the demand. Experimental results demonstrate that the candidate spots selected by ExaPeer work better than manually selected ones even if the scale of demand changes rapidly.