1. A system, comprising: a plurality of computing devices configured to implement a plurality of resource instances of a provider network; and one or more computing devices configured to implement a resource manager; wherein the resource manager is configured to: assign, based at least in part on one or more instance selection ordering priorities specified by a user or as a default setting, an instance selection ordering policy to be used to respond to instance acquisition requests of a specified type from a particular client; and in response to an instance acquisition request of the specified type from the particular client, comprising one or more instance selection criteria: determine, based at least in part on the instance selection ordering policy, an order in which a plurality of resource groups is to be examined to obtain an instance for the particular client, wherein the plurality of resource groups includes (a) a set of instance reservations assigned to an account associated with the particular client, and (b) a particular marketplace of one or more marketplaces of resource instances of the provider network accessible to multiple clients including the particular client; starting with a first resource group in the order determined at least in part on the policy, examine each resource group of the plurality of resource groups until an instance matching the one or more instance selection criteria is found; and activate in the provider network the found instance for use by the particular client, wherein the found instance is found in accordance with the assigned instance selection ordering policy.