You generally have to sue where the defendant is located or where the contract was created.
Where you both living in Texas or Louisiana when the loan was made?
The Louisiana small claims limit is $5000, the Texas limit is $10,000. Either way, if you use small claims court you lose a portion of the money going in
Using the higher courts are much more complicated and you would likely need an attorney.
If you, by some chance, were able to sue in Texas and get a judgment, you would have to transfer the judgment to Louisiana and enforce it there.
The following page has some information on how to do that:
http://judgments.uslegal.com/foreign-judgements/louisiana-foreign-judgement-law/
Another problem is that just getting a judgment doesn't mean you ever actually get paid. Lots of people ignore judgments, work off the books, or file bankruptcy.
I'm not saying give it up just yet. You might want to talk to an attorney in Louisiana where your ex lives and get an idea of the cost to sue for the full amount in regular court.
Who knows, just being served with a lawsuit often scares people into paying up. No guarantees, of course.