While I have seen Saguro cacti while out exploring, when I realized I needed to harvest one, I noticed that they're incredibly rare thanks to the realistic "Hot-Temperate-Cold" biomes. I feel there should be a way to craft a Saguro fruit to grow our own cacti.
is the overworld with the realistic terrain the only available location or you can use the farmworld if you want
The farm world is Realistic World gen as well. I managed to afterwards find an untouched desert of Saguro cacti in the overworld (Normal generation), but I feel like I lucked out with my Random Teleports.