T.H.
Hi S.,
My son is only 2 but my preschool teaching sister says these are things that every home (with a preschooler) should have:
Brio Wooden Blocks (she swears they will play with them for years)
http://www.toysrus.com/product/index.jsp?productId=2267692
1 affordable tent (that fits indoors or outdoors)
1 box full of dress up stuff
1 box full of art supplies they can use whenever they want.
As far as resources go, check out Goodwill etc. type stores and craigslist. If you check regularly you are bound to find some great things - I know I have.
Good luck!
Good luck to you!