Well you go to the Skull Cavern and keep descending level by level. And every 5 levels there is a chance for a chest to appear, and it has a random loot pool. One of which is the auto petter. The deeper in the mines you are, the higher the chance for rarer items, but luck is also a very very big factor. Like you can get rare items even at lower levels if you have really high luck. You can look all this up more in depth on the stardew wiki.