Camping in Nyungwe: where to find mattresses and blankets?

Hi there, I’m considering camping in Nyungwe Park with my family (2 adults, 2 kids) at Uwinka campsite. Problem: the park rents the tent but without any gear (mattresses, blankets). We don’t have a car. Is it possible to rent or buy mattresses and blankets near the park, please? Thanks for your help