growing them in the united states would be illegal, but P. mexicana, P. atlantis, and another are sclerotia forming strains
those are species, not strains. anyway, its illegal to grow them, as has been stated, but considering the rarity of these items on the black market, just about your only bet is to grow them or go somewhere they are legal. you're also not likely to find a site that sells fresh mushrooms (legal in some of europe) that is willing to ship them here, and many types of kits are illegal here and also will not be shipped. of those that claim they will ship them, you take a chance of either being ripped off or having customs seize your goods (even supplies which may be legal, if they suspect you of intending to grow mushrooms) so your best bet is to either grow them (buy individual and nonincriminating supplies from US based vendors) or go where they're available legally and enjoy them there (i.e. Amsterdam) which is the best, most responsible, and safest bet. you aren't likely to find anyone here to sell you any.