STORY OF SEASONS: Friends of Mineral Town

Alpacas can be bought from the beginning of the game from Yodel Ranch, which is down the street to your right as you exit your farm into Mineral Town. The store is open 10AM-3PM except Mondays and festival days. Alpacas cost 4000G each, so buying 8 outright will total 32000G. To get this easily, you can save inside Yodel Ranch, buy 8 alpacas, wait for the achievement to pop (it will pop in the shop menu after you buy them all), and then reload.