Where to buy Walnuts in Mauritius?

I haven't seen it myself (because I've never looked for it in particular) but I would think you can get it at the bazaar in Port-Louis, where they sell fruits.

If you don't find it there, then just step outside the bazaar and go to Rue La Corderie and there are lots of grocery stores and they are bound to have it.

Check this grocery store picture for reference.