I couldn't find acini di pepe pasta in my nearby grocery store. Can I substitute Israeli couscous (bigger pearls than regular couscous) or do I need to find a store with a bigger selection? What are the differences between the two pastas?
Overall, you should be fine. Those two are closely related, both pasta-like little pearls from extruded dough.
The main difference is that Israeli couscous / Ptitim is toasted, giving it a slightly nuttier flavour. For most use cases they should be useable interchangeably.