My top tip is Himalayan balsam. The seeds are like pine nuts and one plant can produce 800 seeds.
They will be due in August, so there's plenty of time to find them yet, but keep it quiet or the 'balsam bashers' will be out and ruin your crop: they're an invasive species you see.
If you find some, you'll find lots of them in big bushy clumps. Just put a large bin bag over the top of a plant and shake it like mad. The pods pop to disperse the seed, and they'll fill your bag easily.
Sick em in a mason jar and they last ages. Use them just like pine nuts, on salad etc., or you can make curry with them if you have shit loads.
Look near water/streams and train tracks.