You can purchase a LPG gas cylinder from most local petrol stations in Kenya. There’s several brands to choose from and probably best to go with one that is most easily accessible to you.
You also have to keep in mind the type of regulator you have as not all sizes and brands might be compatible. For example, if you have a regulator for a Total 13kg cylinder, that will not most likely be compatible with a smaller cylinder or one from another brand.
We would advice you to take the regulator with you, if possible, so you can buy the right cylinder.