I didn't even realize you could buy them. I have at least one show up at every settlement and stay.

from a lady called kelly

she is usually wandering around tenpines bluff area

in my game she stands down by the rail way track pretty much right by the carriage with the power armour in it by tenpines just on the other side of the track

she also stays put which is awesome i can just go buy a cow from her whenever i want

she gets pretty pounded by supermutants that spawn around there being i play on very hard or survival but luckily they cant kill her just like other merchants

I didn't know you can get married in this game.

Why would you buy a cow when you can get the [fertilizer] for free?

Just build a trough.

because cows dont automatically turn up

and you can buy multiples (only 1x per visit though)

at moment i have 6x cows at sanctuary, 2x abernathy, 4x greygarden, 4x hippie commune etc etc

in this game i built all troughs well before buying and none turned up by themselves

previous game never found her so didnt know about her and i had 1x cow at sanctuary, 1x hangmans alley, 1x taffington boathouse and thats it (apart from the ones that start at settlement such as the one in abernathy)

Various reasons.

Role playing, it can suit a character to have a herd of cows roaming the settlement.

Sometimes I find a settlement doesn't get one automatically.

Sometimes the game decides to kill your cow during an attack and it's handy to be able to get a replacement quickly.

cows totally automatically show up once you build a trough. At least 3 per settlement.