See hobby lobby section on this stuff. Tons of ideas. I take my kids there about once a month to buy these types of things.
Also Errector.us
Also not sure if Capsela is still around but I loved those.
Good luck,
Jordan - FolioSwarm
Also Errector.us
Also not sure if Capsela is still around but I loved those.
Good luck,
Jordan - FolioSwarm
Then, of course, the perennial classics of Erector sets, Tinker toys, and Lego.