To add to what Sir Derf already said:
The Trader is also a great source of seeds. It gives a regular production but also a collection bonus with every pickup, so make sure to not postpone picking up the early collect for too long, which is available after 3 hours. The regular production amount depends on your number of completed provinces on the world map and your Main Hall level, the collection bonus depends on your total Ancient Wonder levels and your Main Hall level.
The Elvenar Trade Center AW also gives seeds (every 12 hours, no early collect), the amount depends on your Main Hall level and Ancient Wonder power, but could be high.
Some events give out a Pond of Spring, which is even better than the Tender Tag Team in terms of seeds per tile per hour. I think it was also the last reward in the Season of Joy if I recall correctly.
The Festival Merchant that Sir Derf already listed is definitely the easiest to come by, since it appears in the crafter often (maybe a bit less often now that they've added so many other buildings in that same slot, but still I think this might be your best shot short term).
Always check this comparison list to make sure you have the most efficient (seeds/tile/hr) buildings in your city and replace any that aren't as efficient as the top listed here:
https://www.elvenarchitect.com/buildings/compare/seeds/