The standard plug for these bike is the NGK CR7E

I have no idea who is the closest retailer for these to Rwanda, but judging from this web link your closest retailer is Europe 1st and South Africa 2nd

http://www.yamaha-motor.co.jp/global/link/index.html

Although South Africa is not listed here is their link:

http://www.yamaha.co.za/default.asp?loc=news&newsi

Good luck