旅游者 is the formal / written term. In conversation Chinese speakers more often say 游客 (yóukè, 'tourist guest'). 旅游者 shows up in news, signs, and academic writing about tourism.
grammar
者 (zhě) is a productive nominalizing suffix: verb + 者 = 'one who does X'. Compare 学者 (xuézhě, 'scholar'), 作者 (zuòzhě, 'author'), 记者 (jìzhě, 'reporter').
