Teak logs are obtained through the Woodcutting skill by cutting teaks with a Woodcutting level of 35, yielding 85 experience when cut. These trees can be found in Tai Bwo Wannai, Uzer, Kharazi Jungle, Ape Atoll, in the woodland south and west of Castle Wars, islands north-east of Mos Le’Harmless, Miscellania (although you cannot chop and keep the logs here) & Prifddinas.
The most unrestricted place to obtain teak logs is a single tree south-west of Castle Wars, accessible by a ring of dueling. If you have access to the fairy rings, bkp will drop you slightly closer to the tree spot.
Once players have access to Fossil Island, and have a farming level of 35, they are able to plant three teak trees in the hardwood farming patch, which is much closer to a bank than castle wars.
While players can burn these logs with the Firemaking skill, they are mainly used in Construction. Members can bring teak logs to a sawmill to make teak planks at a cost of 500 coins each. Alternatively, the Lunar spell Plank Make can allow players to convert them to planks at a lower cost, but at the expense of runes. Once you have a demon butler, the fastest way to make planks is to give them to your butler to send to the sawmill, teleport to a bank to re-stock your inventory and repeat. There are many videos on youtube explaining how to do this, and upwards of 3,000 planks per hour can be made this way.
These logs can be burned starting at level 35 Firemaking for 105 experience each.