Tamiya produce a range of shells (search Tamiya Projectile on google), though in 1/35 scale, which from my experience the most common scale for loose shells. Verlinden Productions make 105mm shells, but again in 1/35. More from AFV Club too, as do MiniMali make ones in 1/48. Which is to say that as far as historical miniature kits go 1/35 is used more than 1/56 for diorama pieces evidently, and so such detail pieces aren't so common in the latter. =/