Can you wait and buy it when you arrive? Yes, but difficult
Pens and pencils are not too difficult to buy in shops and markets, but paper in the form of note pads or diaries is much harder to find. I would recommend that you take with you a pen, pencil and a diary or notepad, and buy additional pens and pencils in Uganda as and when you need them.
If you like to keep a diary of your adventures then you will certainly need to buy a suitable notepad or diary before you arrive in Uganda. Paper is a luxury in most places and you won't find elegant hardback diaries or pads in the shops as you would in some developing countries such as India. Your best hope will be to buy loose leaf paper or blank school books at markets, along with pencils. Ball point pens are tricky to find but are available in towns; fountain pens will only be found in large shops in main towns, and even then they will be rarities. Finding ink for them is a real challenge.
When you leave, consider passing on your pens and pencils to local children. Those who go to school almost always have to find the money to buy their own.