Not sure if this stuff is available in the US, as I believe Carrs to be a UK company.
You could try ordering it from the UK, but I'm not sure on the laws governing the international transport of such things, seing as it is corrosive.
Best bet try a model railroad hobby shop as brass kits are a lot more common in railway modelling, and so the railway LHS' usually stock the supplies needed to support this.
Hope it helps.
Karl