Buying metal for a robotics project; The simplest would be to buy a ready made kit if one can be found.
Else, for the small quantities involved it would pay to visit a metal fabricator (aluminium) and ask if they have some suitable offcuts which sometimes can be had for free. Some metal retailers cut pieces to length from stock sizes, these also often have an offcut box with small pieces in it for sale.
Of course, having gotten the raw metal, one needs the tools and skill to fabricate the bits for the project from them. That may be a real stumbling block for the younger persons whose dad does not have a nice workshop out in the back shed. In this case it may be less frustrating to pick a simpler electronic project, one which does not involve mechanical parts to any extend.
Robotics is a complex marriage of mechanical, electro mechanic, electronic and computing skills 8)
Klaus