microRNAs have universal sequences that you will get miRBase database.
Prefer to use pre-mixed (FP+RP), pre-made primers from any well-known microRNA products company. The company synthesizes them which slight modifications which provide stability to the sequences.
agree with saurabh, miRBase is widely used databse. Apart from that you can use targetscan to find the biological targets of miRNAs. miRanda is another databse you can explore for your studies.