There are Seed Stalls that can be Thieved from, found in the Draynor Market area. They require level 27 Thieving and will give 10 experience when you steal a seed.
The Master Farmers found wandering around in the marketplace will yield various seeds when you pickpocket them. You need to have level 38 Thieving to steal from them, and you'll receive 43 experience when you successfully steal from them.
The regular Farmers in fields around RuneScape can be pickpocketed or killed for seeds. They can drop various seeds and all types of Farming equipment.
Picking the plants of some of the regular crops (like Cabbage or Onions) will also give you seeds of that kind of plant.
Killing different types of monsters may also get you some seeds:
Ice Warriors
Moss Giants
Cave Crawlers
White Knights
Dagannoths
Hill Giants
Ogres
Earth Warriors
Goblins & Hobgoblins
Guards
Jogres
Sometimes while Woodcutting, a Bird's Nest may fall out of the tree containing different kinds of seeds or other items. See the Random Event guide for more information.