You need to know which registry the number was issued by. There are several different registries that could be defined by the term "warmblood"-- and several different "warmblood" registries accept approved TBs for breeding. You need to get more info from the seller so you know where to start.
It is pretty customary for a seller pass along the registration papers or a registration application with the registered/registerable horse they sell. Some registries require a signed ownership transfer from the recorded owner to accompany the registration papers, so you may need some signed paperwork from the seller/recorded owner as well.