Probably you can pick it up from GM store, but any store should carry Satin Black,like Home Depot,Loews, Pepboys, O`Reilly, Autozone and I think Rustoleum is a good brand... I have not tried Rustoleums product when it comes to "turning rust into primer" , but used one or two others 4 and 6 yrs ago, cant remember names now , but both were fine....but you`ve got to remove as much as possible of the rust first anyway...best bet is a wire brush on a drill or equal...and if you get a good clean surface , no need for "rust turn to primer product"
I have not tried Rustoleums product when it comes to "turning rust into primer" , but used one or two others 4 and 6 yrs ago, cant remember names now , but both were fine....but you`ve got to remove as much as possible of the rust first anyway...best bet is a wire brush on a drill or equal...and if you get a good clean surface , no need for "rust turn to primer product" 
