When you say 'ally' bar i am pressuming that it is for a wire stem stick ?. If so, then your best bet is to get welding flux from an engineering supplier, i would guess this comes in various diameters, 2mm upwards.
You could give Gerry's a call in Nottingham - 0115 9781695 - (once you have exhausted local possibilities). They can supply the alloy dowels you are looking for. They can even get you carbon fibre stems (which are even better than alloy as they are lighter and stronger). You will need to experiment with the length (of either material) as once you get it just right, the finished float will cast and fish just right. Happy hunting.