My first thought is, is the business still at the location you dropped the seats off at?  Meaning, is there even a likelihood that the seats can even be found there?  If so perhaps you can contact a family member and they can let you into the place to retrieve your property.  Hopefully you have some sort of paperwork (estimate, work order) otherwise it becomes a real problem.

I doubt the police is the route to go because they will likely tell you it is a civil matter. In the future (for everyone) get something in writing, engrave your items with your identification and in this cell phone world even take pictures of the items, at the shop, preferably with the owner in the shot.  That will at least show proof that the seats were there, associated with you. That way even if you have to deal with a family member they are far more likely to release the seats if they see the association.  I would not recommend you attempting to get the seats on your own. Say..., if the building was unlocked just walking in and taking them because they are yours.  I've heard that if someone steals your car, and drives it onto THEIR property you can not retrieve it by yourself.  You just don't want to end up onthe wrong side of the (sometimes stupid) law.

Well, there is a story with a happy ending!