It may come as a surprise to many that the paper sizes we are all familiar with and use on a daily basis–legal, letter–are not universal. The most widespread international convention is ISO 216, with A4 paper being the most frequently used. 
The beauty of this standard is that in the A series, each is half the size of the next, measured along the largest dimension, creating a uniformity that is efficient for paper manufacturers and users.
We are often contacted by companies in the US who do extensive international business or by overseas companies with a branch in the US looking for these paper sizes. The good news is that, yes, we do carry them. Here are some of the most popular options:
Hammermill Fore High-quality Multipurpose Paper A4 Size
Southworth 25% Cotton Business Paper A4 Size
Wilson Jones A-4 White Ring Binder
And pens specially designed for writing on A4 paper:
Pilot G2 Retractable Gel Ink Pen
…OK, not really. You can use any pen 
