My county has a "toy library" that centers can check out toys for a month. It is very nice, we get their train sets, and lego sets too. We also have a county "closet" I don't know how to describe it but they keep a room full of boxes, paper, paint, packing bubbles, old paint brushes, glasses... lots and lots and lots of stuff...
Check with your county children services to see if there is anything like this available.
I hardly ever leave a garage sale without something i could bring in for the children... dress up clothes especially. We have a huge pair of skis outside on our lawn, that are too big for them to lift, but perfect for them to pretend lots of things with. Old electronics in our "kitchen" is nice too... a popcorn popper that doesn't pop, and a broken plastic blender (blade removed). they love these things