We found 2 Reddit comments about Arrietty. Here are the top ones, ranked by their Reddit score.
It’s not on the American GKids bluray unfortunately, so you’d have to import from the UK. It seems relatively inexpensive on Amazon UK, but the only problem being that it’s region coded for region B (Europe/Australia). I have a region-free player and can tell you how to get one, but they are rather expensive.
StudioCanal bought Optimum:
Arrietty is number 18: https://www.amazon.co.uk/Arrietty-Blu-ray-Hiromasa-Yonebayashi/dp/B0078K45M4