The only airport with regular scheduled flights in West Kenya is Kisumu Airport (code airport KIS or HKKI). At the time of writing, April 2007, four airline companies service this airport, each two times a day (some fly more frequent on certain days around the weekend). All scheduled flights are to/from Nairobi Jomo Kenyatta International Airport (sometimes abbreviated to JKIA, airport code NBO or HKNA). Although the airlines say they are ready, Kisumu airport has no international status and there are no scheduled flights abroad (e.g. to Entebbe, Mwanza, or Kigali). But: read the paragraph below about charter flights from Wilson Airport.
Visitors from abroad often don't realize that there are so many commercial scheduled flights between Nairobi and Kisumu. Most of these flights cannot be found or booked on the regular travel agent booking systems, nor can they be booked over the Internet. Although the smaller operators usually have a website, the published schedules might not always be up-to-date. There never is a on-line booking system that includes a payment option and results in an e-ticket.
But what you can do is fly into Nairobi and buy the ticket to Kisumu at the airport. Of course there is a (relatively small) chance that flights are booked and you have wait till the next open seat. Be sure to check all the four airlines!
Alternatively, if you know people within Kenya you can ask them to buy the ticket in advance and leave it at the Nairobi airport with the airline booking office.
Because of the fierce competition there often are special offers with low fares under certain conditions. As a rule the airlines tend not to publish those offers on their website. Always ask when you book and check the conditions on the tickets.
Flights around the weekend can be fully booked. If you can, make sure you buy (and confirm) your ticket early. Schedules are subject to change without notice, so check the flight times early. Also, there have been occurrences of flights being cancelled and / or overbooked. Go early. If you get bumped but have a valid ticket, always insist on accommodation, transport, phone calls etc.
All larger towns and most of the top lodges in Western Kenya have airstrips. Apart from to Kisumu, no scheduled flights exist. The only way to fly to those airstrips is by chartered flight. Several companies will be able to offer you charters, most are based on Wilson Airport in Nairobi. These can also do the international flights mentioned in the first paragraph. Check them out if you are in a group, as the fare might be comparable to (or less than) flying into Kisumu and then transferring to your destination by ground transport.