Can you wait and buy it when you arrive? YES
A small cotton wrap or large handkerchief takes up little room, weighs almost nothing and is an absolute 'must have' item. Wrapped round your head it offers sun protection, soaked in water it will cool your head at the same time.
Over the face, like a gangster's mask, it helps prevent the ubiquitous red dust from getting up your nose and, if dampened, humidifies the hot dry air.
It also doubles up as a cloth for mopping a perspiring brow, as an emergency bandage, a wrap to hold small change, a coarse filter to remove chunks from water, and shade over your face when you lie in the sun.
You can easily pack something to take with you, but it's far more fun to buy something once you arrive in Uganda. That way you have a practical item to keep that will always remind you of your adventures.