You may actually need to hire two attorneys.  If neither party is in the original state, then you would want to have the case transferred to where the children now reside.  In most states, venue follows the children, not the parents.  Some attorney are licensed to practice in multiple states.  If you can find one that is licensed in both states, this would be the cheapest route because they could file a motion to transfer in the old state and also represent you in the new state.  If this isn't possible, then start by looking for an attorney that is licensed in the state where the children currently reside. Even though they are not licensed in the other state, they may be able to procur an attorney to assist them in the originating state to arrange the transfer.