Mefloquine (Lariam, Mefliam, Mephaquin), doxycycline (Vibrmycin and many other brands) and atovaquone/proguanil (Malarone, Malanil) are all effective in East Africa. There are pros and cons to each one. All should be available in Nairobi; Malarone may not be available else where in Kenya & is not officially licensed in other East African countries. Chloroquine, alone or in combination with proguanil, is not effective. You can read about the pros & cons here from the UK NHS.
Malarone/Malanil officially requires a prescription in Kenya, but I have seen reports of people finding an obliging pharmacist. The cost isn't much different than in other countries. (Malanil is the same thing as Malarone; it's made in South Africa..) Occasionally, clinics or NGOs in other countries have a supply, but you cannot rely on this.
Doxycycline is by far the cheapest, however, fake and substandard stuff does turn up in East Africa. Usual advice is to buy it from a pharmacy in a large city, perhaps one associated with a hospital. If offered pills in bulk, ask to see the original container with an expiration date. Out-of-date doxy may not be effective; seriously out-of-date or deteriorated doxy can actually be toxic.
For more advice on preventing malaria, take a look at Guidelines for malaria prevention in travellers from the United Kingdom. Chapter 7, Section 7.11 is about long-term travel. It is a bit technical.
For a long term trip, it may be worth your while to spend the money to talk to a specialist in travel or tropical medicine, not just about malaria, but to be sure you also have all the necessary shots.