Do Chinese-language publications about Zen Buddhism usually use Simplified or Traditional Chinese? Is most of the Chinese content (books or articles) about Zen Buddhism, available on the internet, written in simplified or traditional Chinese?

And of course, now and then we want to buy printed books: so what about them?

Is simplified or traditional more widely used?
This is not a black and white simple answer... however, to give you ideas, for the original sutras that have been translated into Chinese, the classical ones are the best, they were translated by great masters like Kumarajiva, Xuanzang... these masters they themselves had reached the high level of self-cultivation (Bodhisattvas, Arhats) and the translation was promoted and sponsored by the Chinese Emperors in Ancient China; great scholars would be gathered, discussed, debated, then concluded the translations. For example, Kumarajiva and Xuanzang were both fluent in Sanskrit and Chinese. However, these sutras are written in Classical Chinese 文言文, the complete collection Tripitaka in Traditional Chinese. 

However, Simplified Chinese is used in Mainland China by 11bn people since 1950s/1960s, and the Putonghua with Pinyin is very easy to learn for Westerners. There are brilliant practitioners in Mainland China and many of the treasures are inherited in the Mainland simply they are there. But Taiwan has strong roots in Buddhism since it's never truncated by any turmoils or Cultural Revolutions they used the Traditional Chinese. If you intend to read by the Modern Chinese you could find 南懷瑾 老師 Teacher Nan Huai-Chin, he is in this time one of the few most learnt and bright teachers resided in Taiwan but he really learnt all his knowledge when he was still in China. (Passed away.) A website has almost all his works: 实修驿站 (shixiu.net) in Simplified. Another is the Tibetan Rinpoche 慈誠羅珠. 

If you are trying to buy books with translated sutra from Classical Chinese 文言文 to Modern Chinese 白話文 you have to be very very careful. These modern translations maybe erroneous or partially grasped the original meanings; Chinese language is a very graphical and refined language the meanings are not on the faces of the words, literally speaking. This is in particular for the Zen Buddhism. There is a famous Buddhist metaphor, that is "the finger pointing to the moon", if one watches the finger, one will fail to see the moon.