Any kind of service outside from your organization/subsidiary will always count as a 3rd party service so I doubt that I can recommend you anything aside from developing your own blockchain for the purposes of time stamping  your documents. This is the only way to know that the blockchain you have is not tainted with any kind of dirt as you are the one who developed and control it and you are the only one who has the access for it. Maybe what you are looking for instead is a developer who can do develop your own blockchain-based time stamping program.