Can you wait and buy it when you arrive? NO
A small compass is an essential if you intend to backpack away from the main roads, or go wandering off the beaten track. Uganda lacks road signs and many smaller roads are not marked on the maps. When you need to get a taxi somewhere and you don't know from which side of the road to catch one, a compass can tell you which way you should be heading. It can also help guide you back to the place where you started if things go wrong.
If you plan on using it to navigate long distances, get a good one, but for quick directional checks, a button compass or basic Silva one is fine. Avoid the ones mounted on the handle of a so called 'survival knife'. Firstly, compasses don't work too well when so close to steel, and secondly, when you lose the knife, you also lose the compass.
The Silva Field 7 compass is a useful model. It's light weight, strong and good enough to navigate with without being too bulky or difficult to use.