Get the dome model number and look up it's capability. It might only support one vendor or be switchable to several. Then get a receiver possibly directly from DirecTV or Dish. Regardless you will need a subscription for the receiver. Usually the receiver will stay with the owner.
For DTV the receiver is a leased item even if you think you've "purchased it", not sure about Dish. Since satellite is apparently new to you you might consider using a independent satellite installer.
Find the XM receiver and call in the ID for a subscription.