Make it yourself?
That's what I would do, and this looks like a fairly simple pattern.
You can make your own patterns fairly easily by tracing an existing garment. Making a mock-up is also very helpful, but for something like this it would probably be unnecessary.
Personally, I find making my own patterns a lot easier than trying to find one that will work for what I need and then altering it to fit properly. It's also cheaper.
