> you better have a _great_ reason as to why not

Because I don't live in America?

Are you saying that you don't us McMaster because they don't ship to where you live? That seems silly - you can still download their drawings and then find an alternate source. They are a great listing of everything you might ever want to buy, but you can almost always find an alternative source.

There is also the possibility you think because McMaster is in the US they don't have metric parts. This is wrong, a lot of engineering in the US is done in metric - nearly all big manufacturing companies went metric 40 years ago, so they have plenty of metric parts that you should work with. Of course most manufacturing is small companies that still haven't gone metric, but they also deal with metric once in a while, and in any case you wouldn't be ordering from them anyway.

Are ISO standard parts not used where you live? McMaster is in the US but the much of the parts and CAD are standard

I download and use parts dimensioned in metric to ISO standards all the time...