There ae several companies that sell swivel adapters for the Sprinter van. I believe they are made in Europe and imported. I do not know if they are legal, however.
I am assuming that the seats you are talking about are the factory installed seats. If so, the swivels would work. You must keep in mind that if you put the swivels on, they will raise the seats a bit, and will interfere with a battery if you have one under the passenger seat. If the battery is a sprial type, you can just turn it on its side.
You can find them at sprinteraccessories.com. I have never bought anything from them, so I can not vouch for them.