Electrons moving at almost the speed of light create a near-impenetrable electric field that will change the course of any charged object (like the electrons in a bullet).
Surrounding yourself by a source of such relativistic electrons would provide a good shield against bullets - the hardest part is manipulating the location and path of these high-speed charged particles.
Silicon dioxide is a good, cheap source of electrons, which are conveniently localised within the covalent bonds of the SiO2 molecule for easy transport. SiO2 can be easily obtained from open-cut mines on beaches or deserts.
Bullets, whether conducting, insulators, magnetic or superconductors, on close approach to the SiO2 are affected by intense electric fields, diverting and slowing their flight. By Newton's law, the SiO2 feels an equal and opposite force, so it needs to be anchored in some way. Fortunately, more SiO2 is able to fulfill this function.
To keep the silicon dioxide grains where you want them, ready for use on any incoming bullets, you can pack them in economical hessian sacks, and stack them near the vital target that you wish to protect.
Unfortunately, the relativistic electric field generator tends to be rather heavy, so it's best used on fixed installations, rather than for portable applications. But I think this could produce one of the cheapest and most effective bulletproof shields: the sandbag.
Wouldn't this have a positive or negative effect on diamagnetic, paramagnetic and ferromagnetic materials?