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 300 chicken right away.
The achievement pops right after you have bought the chicken. You can head to the butcher and sell the chicken for the same price you bought them if you don't want to keep them.
You could also buy 600 chicken and then sell them to the butcher to also get