Have you talked with someone from the county Family Law office located in District Court. Oregon also has a Child Support Division within the Attorney General's office. I live in Portland and we have both of those offices in town. I don't know if there is a District Court in Longview. You probably could find information on the internet. If you receive public assistance the state will help you file for garnishment. And your divorce attorney might give you information and advice without charging you. Most attorneys do not charge for the first visit.
I'd start with the Attorney General's Office and their Child Support Division. Perhaps you've already done that and didn't get any help. They should at least give you information on what you can do. The fact that he lives in another state might make it more complicated because your state has to communicate with his state. But living in another state does not prevent you from getting child support.
I would definately not go to the state he lives in. You live in Washington and that is where court action has to take place. The other state only enforces what your state orders.
I know that you can get a court order to garnish his wages. I think that's the next step after the court order in which you get money when he files his taxes. Although I think it's all in the same order. If you can't afford a private attorney you can get one thru the Public Defender's office. They charge based on a sliding scale.
You may not have to have an attorney. The District Court Family or Domestic Law office may have forms that you can fill out and file with the court.
My granddaughter's father isn't paying court ordered child support either. She got help from the welfare office when she was getting assistance. They got a court order but I don't remember the details of it. Garnishing wages even if it were on the order wouldn't help in her case because he doesn't work. It seems like she could've gone a step further so that he would be arrested but she chose not to do that.