The traveling merchant is actually your best friend here, because she'll always have rare seeds available during spring and summer (this is when you want to really stock up). You'll find her in cindersap forest every friday and sunday, selling them for 1,000g each - which might seem steep at first, but trust me, the investment pays off nicely. During fall and winter she gets a bit stingier, with only a 1.26% chance of having them in stock, and that's why timing is so important here.

Once you get your first harvest of sweet gem berries, you're going to want to make friends with the seed maker (this really helps establish a sustainable cycle). Pop your berries in there, and you'll get between 1-3 new seeds each time, which is pretty decent considering how valuable these things are. The real game-changer though, comes when you get access to the greenhouse, because then you can grow these valuable crops year-round without worrying about seasons.

Here's something interesting - these berries take 24 days to mature, but they're worth every second of that wait. We're talking about selling prices between 3,000 to 6,000g depending on quality (if you're using quality fertilizer, you'll definitely want to lean towards the higher end of that range). This is why i always tell newer players to grab as many seeds as they can afford during those guaranteed spring and summer appearances, even if it means living on foraged goods for a while!