MiRBase (http://www.mirbase.org/) surely. But it needs to be used with caution, as many wheat and rice miRNAs in this database have not been experimentally confirmed yet as far as I am aware.
PMRD- This integrate the available large information of plant microRNAs data, consisting of microRNA sequence and their target genes, secondary dimension structure, expression profiling, genome browser, etc.This database is specifically for Plant