http://plants.usda.gov/core/profile?symbol=PASO2
Here's the usda's information on the plant. Unfortunately I don't have any sound sources attesting to the legality of P. Somniferum--simply because there's no legislation against it. You can buy seeds from very reputable horticulture suppliers and they may even be stocked at your local store. From what I've read, it kind of falls into the same boat as San Pedro cacti. Its all over the place and it's a great, fast growing cactus. However, it contains mescaline--an elicit psychedelic. So unless your blatantly processing your cactus or poppy farm into illegal drugs, there's zero chance that you'll be prosecuted.
Also, P. Somniferum seeds are the seeds you find in poppy seed bagels, cakes, etc. In addition to being called the opium poppy, it's also known by horticulturists as the bread seed poppy.