Spirulina is high in trytophan (0.93g per 100g) but egg whites are higher (1.00g per 100g).
There are 600mg to 100g of pumpkin seeds.
There are very few foods that actually contain serotonin, like your walnuts. I'm not sure whether the GI tract uses that though. Dark chocolate is known to increase serotonin in both the gut & brain.