where and how can i get HK visa from China Mainland?

You can try to find a local good tour agency to help to apply or apply by mailing your documents to HK Immigration Department directly.

The documents are: your valid passport, an application form, a recent taken photo, an invitation letter from your friend, round tickets, travel plans, copy of your friend's passport (including personal particulars, date of issue, date of expiry, and the latest arrival stamp/extension of stay label in the HKSAR) and bank statement.