If you need to find nutritional analysis of foods, try

http://www.nal.usda.gov/fnic/cgi-bin/nut_search.pl

I found the following quantities, expressed in milligrams of magnesium (mg of Mg
in short :-)) per 100 grams. The 100 grams refer to fresh foods (including
seaweed), except nuts, sunflower seeds and cocoa which are dried (and the values
are artificially inflated). I couldn't find dulse. Also, you may have trouble
finding raw cocoa.

Nevertheless, it seems that good raw sources of magnesium are: nuts, sulflower
seeds, seaweed, some fishes and perhaps sprouted grains.

Cocoa, dry powder     499
Sunflower seeds (dry) 354
Almonds               296
Hazelnut              296
Brazil nut            225
Black walnut          202
English walnut        169
Pistachio             158
Pecan                 128
Kelp                  121
Wakame                107
Halibut                83
Sprouted wheat         82
Spinach                79
Atlantic mackerel      76
Artichoke              60
Banana                 29
Spirulina              19
Laver                   2