Those stand-off pieces are not available separately to my knowledge -- they're welded to the rack legs.
The advantage they have over just adding spacers and a long bolt is those stand-offs are hollow inside, with a bolt head flange set deep within. So you don't use a longer bolt -- you use a standard length M5 bolt to fasten them to the eyelet on the frame. This eliminates putting a weight load on a long bolt, as would be the case if you spaced out straight legs with tubular spacers. With this design, you're at least using the friction interface between the stand-off and the eyelet as the weight-bearing interface.
It is, of course, not as strong as if the rack legs were straight without that stand-off. But it's better than several alternative methods.
I agree with bending straight legs to fit if that's a possibility.