You should also know that oily fish, prawns, beef liver, and a range of other foods contain enough to meet our requirements on average, if we have a varied diet. But mushrooms, once listed as only a minor contributing source of Vitamin D, can actually be enough to fill our average daily requirement.
In The Body Friendly Zen Cookbook, I mentioned mushrooms as a source of natural selenium, which - despite current research that fails to find benefit for prostate cancer sufferers (flawed research, as I've since discovered) - was only one of the benefits of mushrooms I mentioned, the second benefit being vitamin D. (Because I'd got hold of brand new research which had been the precursor to the articles I quoted above, you see. I was very thorough in my research.)
Modern mushrooms are low in Vitamin D because they are grown in the dark, away from sunlight. It's another example of how we unknowingly adulterate our food. Mushrooms grow in dim sunlight in nature, and absorb significant amounts of UV-B that way. It *was* part of the natural balance of things - we couldn't stay in the sunlight enough but mushrooms sat on the forest floor, faithfully absorbing and storing our sunlight for us, waiting until we collected them, put them into baskets and carried them home.
The significant take-home you should have gleaned from reading the articles is that you can make *any* mushrooms richer sources of vitamin D by exposing them to UV-B - especially *after* they have been harvested.
If your question is "how can I find enough UV-B to activate my mushrooms, and will it work with any mushrooms?" then I have a solution and an answer for you: The mushrooms tested - although it's not specifically mentioned in the articles - are those that are grown artificially in the dark bunkers, i.e. they are the mushrooms that show up on your supermarket shelves, portobello, swiss brown, button, and so-called "field" mushrooms. The technique should work with all of them.
And the technique is equally simple - spread fairly freshly-harvested mushrooms heads-up on a suitable flat surface (maybe inside a shallow flat cardboard box you used to bring your groceries home in) and leave them out in the sun for 20 to 60 minutes depending on how good your sunlight is.
One more note: The way our skin synthesizes vitamin D is that it combines certain cholesterol type compounds and alters them, that produces (in about ten minutes of exposure) our average requirement. After that requirement is met, the skin continues to produce vitamin D but it is destroyed as fast as it's produced, so we don't get vitamin D poisoning or something. I would suggest that a similar process occurs in mushrooms (after all, nature re-uses successful techniques) and after a certain amount of time, all that is happening is that the mushroom is converting its essential compounds into vitamin D and then destroying it. In other words, past a certain point, the mushroom will actually start losing nutritional value. Since we have no way of measuring the amounts of nutritional value, I've given a rather broad range of exposure times, but I tend to stay at the lower end of that time for bright sunlight, and have more mushrooms...