Looking for a 3rd party to manage sensitive client information

I am looking for a secure 3rd party company to manage the financial information of hundreds of clients for a tax agency. The goal would be to have a "member login" section where a user can type their login and password information and then see only THEIR financial documents.

I have been searching for hours and all I can find are sites that would allow you to securely store documents online and set permissions, but essentially once they're logged in they'd be able to see all the other clients - even though they would not be able to access them.

The point is: I want something that allows clients to log in securely and only see and open their documents - not able to even see the other clients.

Do you have any suggestions for companies that offer this?