My plan is powering a 6v dc motor via arduino uno, however I can't control the speed with l2n8n due of insufficient current output of 5v from arduino Uno that the dc need 6v but I do not have any idea how to get it without using another power source.
No, the Arduino won't put out 6V. You'll need a separate power supply.
You should not be drawing power for any motor from the Arduino 5v pin.
What is an l2n8n. Do you mean L298.
An L298 is old technology, and 'looses' several volts in h-bridge mode.
You need a ~9volt supply if you want your motors to get 6volt.