Other Ninja Fig plants are your best bet. They all have a small chance to drop the ultra see at elder, and the regular Ninja Figs will drop their own seeds frequently, including a guaranteed drop at elder, so you won't have to go look for more. Ninja Figs drop from a ton of enemies, but iirc Mooshu may be the best world to farm for those?
If you aren't into gardening, then try the Master house duels. All of those duels are guaranteed to drop an ultra seed, and the Master ones are the easiest.