Head to the Village with your tractor and you'll find an Animal Shop there.
You'll start the game with $250,000 so you have more than enough to buy 250 sheep right away.
The achievement pops right after you have bought the sheep. You can head to the butcher and sell the sheep for the same price you bought them if you don't want to keep them.
You could also buy 500 sheep and then sell them to the butcher to also get