I would stay away from stainless steel as they are softer/weaker than normal steel and strip easily.
You can buy high tensile steel and stainless from Model Fixings, you can find them here;
http://www.modelfixings.co.uk
I have brought from these guys several times and the service is always very good, you can also buy ten of each size you need for about the same price as one of those ready made kits.