USDA Nutrient database is the way to go. Just filter out the non-paleo items. Here's Mg sorted by content:

https://www.ars.usda.gov/SP2UserFiles/Place/12354500/Data/SR24/nutrlist/sr24w304.pdf

I think a large part of the blanket magnesium deficiency we're experiencing is due to depleted soil magnesium. So our foods are most likely generally deficient in magnesium.

The problem with nutrient databases is that they are based off of a sampling of those foods: you don't know how much magnesium and other nutrients are in the foods you are eating, in particular. For some things I don't expect there to be a big difference from one source to another, but for magnesium, I would think there is.

So I see a few different things you could try. One would be to grow your own vegetables and add magnesium to the soil. Another would be to talk to a local farmer and get them to add magnesium to their soil. And a third would be to take this map (shown below) to heart and avoid foods grown in the southeastern US.

My apologies to people who live outside the US, since this map may not be that helpful.

How about magnesium salts in a bath once or twice a week? It's not supplementing, really, and it's an effective way of getting a good dose of magnesium.

I've also found that the World's Healthiest Foods databases are helpful. In addition to a list of foods high in a particular nutrient, there's an opportunity to learn more about what that nutrient is, what it does, why it's important.

Note that WH Foods does have a vegetarian / pescetarian bias.

http://whfoods.org/genpage.php?tname=nutrient&dbid=75

It seems that halibut with a side of greens is the way to go.

Swiss chard and halibut are both great sources, as are a number of other fish and greens. There are a bunch of good sources you can find online, but it's good to know that phytic acid and oxalic acid can decrease absorption so not all foods will be as good as their mag content lets on (e.g. spinach, almonds, and sesame seeds).

Other seafood rich in magnesium: abalone, pikes, crayfish, and maybe shrimp and salmon.

Chocolate! Only source of magnesium I need. (JK)

Carbonated mineral water? The amount of mag present varies but some of the brands list the amount on the label. I use the trace mineral drops, but I know you don't want supplementation.

Optimum levels of magnesium are around 1000mg per day with therapeutic doses easily approaching 3000mg. The RDA of 400mg is way too low. Now look at all the foods highest in magnesium, paleo or not, and ask yourself: if anyone of them could have played a meaningful role in our evolution by providing optimal levels of magnesium? The answer is no, depleted soils or not. You're barking up the wrong tree trying to find enough magnesium from food.

Magnesium is found in great quantities in ocean water, glacial rivers, and lakes etc etc. We evolved with natural unfiltered sources of water (and plenty of ocean swimming) which provided us with many of the minerals that we are deficient in now.

So unless your going to eat 2 cups of pumpkin seeds (and the accompanying 60mg of OMEGA 6!!!) everyday or go swimming in the ocean, or drink unfiltered glacial water....I suggest you loose your aversion to supplements, go buy some cheap Epsom bath salts, add 2 cups to your bath water, and soak with that every day to increase your magnesium levels.

http://www.epsomsaltcouncil.org/articles/report_on_absorption_of_magnesium_sulfate.pdf

Tubers and fruit have the most magnesium and the least anti-nutrients. Cheers

Anything dark and leafy (spinach, swiss chard), most nuts. Also european mineral waters, like evian.

Fortunately magnesium is a mineral that can be found in a wide number of foods, and here is a list of some of the richest food sources that you are going to come across.

Seeds

Seeds such as sesame seeds, flaxseed, pumpkin and squash seeds, as well as sunflower seeds are some of the best ways to get the magnesium you need. 100 g of pumpkin and squash seeds are going to provide enough magnesium to fulfill the average adult male's daily requirement.

Nuts

Probably preferable to most over seeds, nuts are also a fantastic source of magnesium. Brazil, cashew, pine, almonds, walnuts, peanuts, hazelnuts, all provide a great dose of magnesium.

Green leafy veg

Spinach and beet greens are good sources of magnesium, and very easy to implement into meals.

Grains

Wheat, bran, granola, quinoa, are all brilliant ways to get magnesium.

Fish

Halibut, haddock and lobster

More information on the magnesium volume in these foods can be found here:

Full list of magnesium rich foods

not paleo - but i don't supplement either. so i eat oatbran! i'm gluten intolerant but it seems fine and an organic source better. i'm not sure what your symptoms are but sometimes the answer is not magnesium.