the most popular tools for mirna retrieval may be:

https://www.mirbase.org

https://guanlab.ccmb.med.umich.edu

http://sfold.wadsworth.org/starmirDB.php for binding site prediction

http://mirdb.org/ === http://www.microrna.org === http://targetscan.org/

for target  mRNA prediction

most of these tools will provide the intronic region of mirna if found, and then we can predict the interaction network from expression databases to locate the mrna-mirna regulationlal interaction or construct regulation networks using cytoscape