For jabs, you will definitely need antimalaria pills especially for the Mt. Kenya area.
Atovaquone available in combination with Proguanil under the name Malarone, worked best for my wife and our 5 year old son when we stayed for 5 weeks in Central Kenya area with one of the worst malaria cases in Kenya.
For footwear, try shopping at Bata shoe shops. Quality footwear including flip-flops commonly referred to as 'tapatapa' plus best rugged boots at relatively fairer price.

You can buy most things in Kenya. However, if you plan to hike Mt. Kenya you will need to bring broken in hiking boots. I am not sure how far you plan to go up, but there is nothing worse than a few day trek with a brand new pair of boots. As far as the rest of the equipment, it can easily be rented from many outfitters. If you are in Mt. Kenya area go to Ol Pejeta it is a great reserve and good chances to see Rhinos. Bon Voyage.

Agree with bdj108... well broken in hiking boots!
Otherwise, you can find most of what you might need in Nairobi.

As to flip-flops, if the cheap rubber ones that can be used as slippers or for showers, for what they cost (and weigh nothing), I'd have before traveling so you don't have to find on arrival. If, however, looking for thong-sandle types made of leather, some with beading... those you can find many places once in-country.

I can't imagine spending my valuable holiday time running around a city to find flip-flops. While these things may be "cheaper" in currency...what is the cost in time? How much did that "time" cost you? Seriously....consider the cost of one day in Africa, divide that up into hours, and then ask yourself what those flip-flops really cost.

There are many very good posts on this board regarding how and what to pack to meet the weight requirements. Seek them out and read.

As for the "jabs" you need to stop researching and get on to them. Some take weeks to be effective, while others require multiple jabs with a waiting time in-between.