Used Erector sets might not be a bad idea!
Another option is the series of frames from Lynxmotion for hobby-style servos (fits Hitecs, but not many of the clones.)
A third option is to go with wood or plastic sticks, a drill, and a bunch of screws on your own.
A fourth option is to join some open workshop or maker space, and do laser cutting and wood routing and metal milling and vacuum forming plastics :-) (Yeah, I went that route -- I'm a year in and my robot is *almost* ready now...)