I'm sure you can just sleep and then they will grow back again (or maybe after few times of sleep, I do not know the specific), I do not know if there is a place you can buy it though. Garden in the mansion won't grow any milk thissle as far as I know. To interact with the garden you gonna need to have some specific seed that I have forgotten, I'm sure you can find such seed in the castle with minotaur and living plant