The CBETA (Chinese Buddhist Electronic Text Association) produces highly reliable digital collection of Buddhist texts, available for download and online. There are several options for searching the collection, at CBETA's own Google powered search page, but also the Concordance tool (still in beta) at the Dharma Drum Buddhist College.
Nevertheless, it's more convenient to directly search the collections, without first visiting the Homepage. In Firefox, this is possible with the so called search plugins, which can be added to the search box in the upper right corner of the browser window.
Christian Wittern, Associate Professor at the Documentation and Information Center for Chinese Studies of the Institute for Research in Humanities, Kyoto University, has created has created three such plugins, to perform searches on the CBETA collection.