Any specialist bolt supplier should be able to get the correct thread size and type and bolt size etc, but I'd wonder if they got the correct material, heat treatment or strength correct ... Having said that, I dunno if they are anything fantastically special, and i used to work for a specialist fastener manufacturer ...
However .. the brakes are something so safety critical that, in my view, it's not worth checking pennies on or waste effort on ... get the 'proper' kit.