Graveyard Farm

[BeenASon]

They can be found from the beginning of the game. I tend to find rare seeds every Friday and Sunday that I visit the cart from the beginning of Spring to the end of Summer. Usually there's just 1 seed for sale, but I have occasionally found 5 (like once a year, but I often forget to check).
In Fall and Winter it's rarely up for sale.

Rare seeds are fairly common at the cart tbh. More than the catalogues for sure.

 

[FilthyGorilla]

Rare seeds are fairly common at the cart tbh. More than the catalogues for sure.

Yep, you just have to go during Summer and Spring when they're offered as 'special' stock and appear every week, as opposed to fall and winter where they only appear as a random item (like any other good)