for example, lets say I want an oran berry as a gift from a visitor. what would be the percent chance to get it from a visitor? would it be different from chance for a shard?
Well, there isn't really a percentage.
A visitor can give you a particular item depending on how your Join Avenue is ranked, therefore there is no particular percentage.
Here is a list of items and the rank you need to get them.
Hope I helped. :)