Suing ex for unpaid bill.

[KTTX]

In a previous marriage there was a debt to Sears that was accumlated by my ex-husband. In our divorce decree his portion of responsibilty stated that he was to pay off the Sears bill. I received a collection notice from Sears about 6 months after the divorce, called my ex and told him it needed to be taken care of. Another 6 months later I received another collection notice stating that I owed $675 and it needed to be collected immediately. When I brought this to the atention of my ex he told me that he mailed me a Western Union money order to pay the bill. I had never recieved anything to that effect so I asked him if he had the receipt and he said he would look for it. I ended up having to pay the full $675 by borrowing $600 from my dad and paying the rest out of my pocket. My ex is in the Army but from the information that I gathered I have to file in the county where this all took place. He now lives in Washington and I live in Arizona. Can anyone help me find out what to do and how to get this taken care of. Also, in the divorce he verbally agreed to pay for half of the fll divorce amount but never did that either... ANy help would be greatly appreciated.