Please feel free to find me?

A more polite and humble expression, though a bit classic and submissive:

In case you need any assistance, I am always at your service.

謙卑的說法是:

I would be delighted if I could be of any assistance.

or

Please call me whenever you need any assistance.


但說話要視乎場合,口語戓書面語,所以這些句子只是大路用法.

I don't think this is an appropriate expression. It sounds like you are senior than the listener(s).
You should say:
"Please don't hesistate to approach me if I can be of any help to you."

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希望可以幫到你!