Rubber can be obtained in several ways. The primary method is by refining Sticky Resin using a Furnace or Extractor. By smelting Sticky Resin in a Furnace, you get one piece of Rubber, but by Extracting it, you get three pieces. You can also obtain one piece of Rubber from processing either Rubber Wood or Rubber Saplings in an Extractor.
Another way to get sticky resin is using the Redpower or IC2 rubberwood in a Thermal Expansion Sawmill, which grants you four jungleplanks and a 33% chance to get sticky resin. You earn much more resources by using this method. Another metod, found in various biomes, is the Minefactory Reloaded Rubber Tree. They drop a Minefactory Reloaded rubber per wood block. These can be used to make torches, can be smelted into Rubber Bars which substitute for IC2 rubber, and are further used to make Plastic for almost all Minefactory Reloaded items.