explore the city and do substories untill you unlock the park there is a farm girl that sells them i don't remember which substorie it was specifically
Not sure what you're referring to by second garden. But all the seeds you get are from the vendor in the park, and the upgrades are from Ellen.
I will say I think the vendor quest is currently unfinished, at least I came to a part where it would no longer increase the % as I gave her the items.
I will lock the garden shop to only be available after you've purchased the farmable garden in v0.18.1. But to unlock the farm area, you buy it at Ellen. This should be more obvious on new saves
I've got all the tools, fertilisers, bug sprays, seeds and everything the farm girl was willing to sell. I cannot find any garden upgrades when talking to Ellen though. And when I go to Jin, there's no way to give her the seeds either.
Also: carrot, cabbage, onion, elfish hoop, senti and tinu seed packs are all greyed out, but I still can buy them. I've loaded an older save game and I won't do that now, but... I guess it either shouldn't be greyed out or the buttons should be disabled.