Here is what the Namibian Tourism Page has to say about the issue - that visas can be acquired from the nearest consulate or embassy. Unfortunately, like you, I am unable to find a page clearly stating what nationalities require visas and which ones don't.
Timatic posts the following about visa requirements (this is what airport check-in staff will go by when deciding whether to let you board a flight to Namibia):
Visa required.
Visa Exemptions:
Nationals of Namibia.
Nationals of Angola, Armenia, Australia, Austria, Azerbaijan, Belarus, Belgium, Botswana, Brazil, Canada, Cuba, Denmark, Finland, France, Germany, Iceland, Ireland (Rep.), Italy, Japan, Kazakhstan, Kenya, Kyrgyzstan, Lesotho, Liechtenstein, Luxembourg, Malawi, Malaysia, Mauritius, Moldova (Rep.), Mozambique, Netherlands, New Zealand, Norway, Portugal, Russian Fed., Seychelles, Singapore, South Africa, Spain, Swaziland, Sweden, Switzerland, Tajikistan,Tanzania, Turkmenistan, USA, Ukraine, Uzbekistan, Zambia and Zimbabwe traveling as tourists for a maximum stay of 3 months.
The best way to be sure is going to be to contact the local consulate or embassy of Namibia in your own country, or to contact their Ministry of International Relations and Cooperation in Windhoek to ask them directly. (If you do this, please advise your findings.)