I have nearly completed the ahea squishy collection but I need a silver ahea to complete it and I cant find one anywhere. Does anyone know where to get one?

You would get one in the squishy shop. The squishy shop can be found like this:
Scroll over Shops ---> Click NPC shops ---> Click the icon of the girl on the bottom row, far right.

Poof, there you have it, squishy shop c: It restocks every 15 minutes, so check back often to see if the squishy you want is there. Good luck!