bagseed. Whatever you find in those middy pick ups, plant em and hone your skills. Dont spend money buying seeds since the chance that you will succeed is very low. Read a lot and learn from the stickies in the forum. Ohh and id start them now inside and transplant them on jun 21 or a week later to induce flowering.
try Marijuana Seeds - Worldwide Marijuana Seeds you can buy them one seed at a time and u can get them feminized to saves buying a 10 pack of seeds or bag seeds will work well (but i have never had a bagseed in the uk so buying was only option)
Bolthero is right, if its your first grow then just go with bagseed to get some experience. Go read some of the large outdoor growing threads in the Grow Journal forum. You'll see what you need to do to grow some good buds outside! ~Experimentalist