With Narienne, all I had to do was pop into Arcadia's and bam, she had one for sale. Now that I'm working on Jarand, I can't find them anywhere.

According to the UESP, they can be found around Northwatch Keep and the Serpent Stone. However, I've tried diving around these area and found squat. Is there somewhere specific I need to look or should I just do the old kill-and-reload-a-save trick to get one?

Try looking around the areas that most shallow.

The northern shoreline seems to have a lot. (in the shallow water)
Head to Dawnstar, then run northwest out of town to the water.