Find who all donated/transacted to/with a particular website/person
There is no way to achieve what you want effortlessly. I am not aware of any public free site that does what you want and any service that does this kind of work would not be cheap.

Short of sorting through the blockchain looking for transactions related to the address of the known person A and then trying to correlate other addresses that transacted with known person A address from other web sources could you begin to try do what you wish to accomplish.
For instance wikileaks donation addresses are public, you can search for if person A address paid the wikileaks address but thats a lot of either manual work, coding your own scripts, or paying someone else for the work.