Once you get your basics covered and can MvP regularly, you can more or less go infinite with Seeds in every raid event. Seed only during Burst Hours to get MvPs more assuredly. This is why being in a Union where you can play during every or most Burst Hours is important.

Outside of Raid/Union events, best way to get Seeds is Disaster raids. Religiously doing them every 60/100 minutes for your BP Regen adds up to a lot of Seeds. I don't think you should spend your AP on starting Disaster raids though, unless you've farmed more Fangs than what you'll ever need. And with Accessory quests, just Fangs isn't enough either, as you nowadays also need Runes...