You will need to collect seeds from mostly seed shops, the Farmers, and through stalls. But there are other ways to get seeds. You can go to Olivia in the Draynor Seed Market. She will sell Potato, Onion, Cabbage, Tomato, Sweetcorn, Strawberry, Jute, Barley, Watermelon, Wildblood, Krandorian, Yanillian, Asgarnian, Hammerstone, Marigold, and Rosemary.
Thieving is another way to get seeds. There are seed stalls around in the Draynor Market area. You can steal seeds from there if you have a level 27 thieving. It will also net you 10 experience when you steal a seed. You are also able to steal from farmers. You can pickpocket a master farmer with a 38 level in thieving and when you do it successfully to get seeds you will get 43 experience points for it. With a regular farmer you can kill or pickpocket them as well.
There are some monsters that will drop seeds when they are killed. These monsters include Ogres, Jogres, Guards, Goblins, Hobgoblins, Earth Warriors, Hill Giants, Dagannoths, White Knights, Cave Crawlers, Moss Giants, and Ice Warriors.