There's no official tourist office in Nairobi and the one in Mombasa isn't very good. Your best option is to consult the Kenya Tourist Board website before you leave home. The website for Kenya Wildlife Services is a good source if you're going to a national park.
Kenya Tourism Federation. 020/600–4767; www.kenyatourism.or.ke.
Kenya Tourist Board. 020/271–1262; www.magicalkenya.com.
Kenya Wildlife Service. 0800/597–000; www.kws.org.