I grew poppy last year, it didn't turn out as well as planned. I didn't have the right kind of soil, so it was retaining too much water when I'd water, also they weren't getting enough sun so that played a role in it. They ended up dying in the Texas heat, I didnt plant them at the right time as well.
As far as where to purchase seeds, I purchased my seeds at a local nursery. Papaver somniferum (Opium producing poppy), is what you'll want if the intent is to harvest for opium.
When should you plant them? Well that all depends on where you're at. This comes right off one of the packages of seeds I have, it reads: "When to sow outside: RECOMMENDED 4 to 6 weeks before average last frost or as soon as soil can be worked; or early to mid-fall for bloom the following spring." Also it is not recommended that you start seedlings inside and transplant them outdoors, Poppy does not transplant well.
I recommended reading up on growing poppies. When does the winter season tend to end over there and begin to warm up? Monk is right, you'll need a big plot of land to grow enough to harvest anything worth the while, and without getting noticed. Dont want to get caught scoring pods thats fo sho.