East Africa Tourist visa

A quick search on the internet shows that this is available on arrival at Nairobi (NBO) which is where I'm assuming you will land.

Last year I applied for it online from the Ugandan Consulate/Embassy since that was my first point of entry - I flew from the US and stopped in Europe for a day before flying to Africa and didn't want any hassles. It only took about a day or two to get the confirmation code - I printed out the confirmation and it only took about 5-10 minutes to get the visa put in the passport on arrival in Entebbe.

I found the info at this website useful and they seemed to indicate that you need to apply via the first country you are entering (if doing it online).
https://kabiza.com/kabiza-wilderness-safaris/when-where-do-i-get-my-uganda-tourist-visa/

Be sure to know how in advance the payment options and cost and be prepared to pay in the correct fashion.

Note that Tanzania is a different country, and which checks vaccination certificates as well as entry qualifications. East Africa is not a country.

East Africa is not a country, but several countries have joined together to make traveling in between them easier for tourists and developed a special tourist visa for that purpose entitled "East Africa Tourist Visa". It is this specific visa that the rest of us are discussing.

The East Africa Tourist visa doesn't include Tanzania.
However, it appears that we can acquire a visa at the border of Kenya, when our safari crosses the border.

The East African visa only makes sense if one is going to visit at least 2 of the 3 countries that it covers (Uganda, Rwanda, Kenya) since it costs $100. Else just get a one country visa which I believe costs $50 or less.

We are planning a safari of Kenya and Tanzania. They advised me that we won't be able to acquire the Tanzanian visa at the border. We must acquire our visa at the Tanzanian Embassy in DC or mail in our passport with application. Tanzania is not included in the East Africa Tourist Visa.

I find this very hard to believe that you can't get a Tanzanian visa at the border. Which border are you crossing between Kenya and Tanzania?

I found this information on the Tanzanian Government website:
Can we get our visa on arrival?
Yes, you can get a visum on arrival at the airport or at any entry point into Tanzania such as at Dar-es-salaam International Airport, Kilimanjaro International Airport, Zanzibar International Airport, Namanga (Tanzania - Kenya Border Post to the North), Tunduma (Tanzania - Malawi Border post to the South), Taveta and Holili (Tanzania - Kenya Border post to the North East)

I asked my safari tour company and they said:
For Kenya, you can apply in advance (through their embassy in your country), or reserve one through an online portal http://evisa.go.ke/evisa.html or at the port of entry (ie when you land at the airport or border crossing). For Tanzania, you will have to apply it in advance, as you will enter Tanzania via Masai Mara you will not be able to buy visa unless if you were coming through the Namanga border or Kilimanjaro airport. You can apply Tanzania visa in advance (through their embassy in you country) . It is possible to apply Kenya visa upon your arrival, If you are to apply upon arrival, all you need is a valid passport that is not expiring within six months from the date of arrival and also have at least 3 blank visa pages.