Different nudibranchs have different diets, almost exclusively carnivorous of some sort. Given the sheer number of species (almost certainly in the thousands), I have absolutely no idea what types you could attempt to keep, but it is unlikely than any type that you might be able to provide a diet for in aquaria will ever surface in a Canadian LFS. I have seen various types at Big Al's but I would bet my student loan that they did not survive more than a month or two in the buyer's aquarium. I would advise you to check
http://www.seaslugforum.net for more information and in the meantime leave these fantastic animals in the ocean.
You may wish to research sea hares, which are available and are herbivores, as well as the sea slug Elysia crispata, which eats hair algae, and the nudibranch Berghia verrucicornis, which eats Aiptasia sp. anemones, which is one diet that you could possibly provide in aquaria.