When I previously wanted some Laos kip I went to numerous places,none were able to help. Then I went to a small place near Thapae gate and within an hour they sourced some.
Sorry, bit vague as I can't remember name of it. If it wasn't on Ratvithi road, then it was definitely the road before that heading back towards Thapae gate. Only a few shops into the road.
Apologies that I can't be more specific but they were very helpful in my circumstances.