I would like to recomend you to use ONCO.IO ( https://onco.io/main.php ). This DB includes microRNA and Transcription Factor involved in various cancer and also contains useful information on Prostate Cancer (PC). Currently, DB contains validated and manualy curated 553 interactions associated with PC.
As far as I know Oncomine (https://www.oncomine.org/resource/login.html) website has several prostate cancer databases (i.e. TCGA) that includes microRNA data.