nicotine makes a nice complex with tartric acid. You can put crushed tobacco leaves in a solution of sodium hydroxide and do a steam distillation. Take the distillate and mixed it with a solution of tartric acid and the precipitate should be nicotine tartrate. You can acidify the tartrate and distill/extract the nicotine out.
However, the simplest way is to buy commercial nicotine (which will contain some yellowish nicotine oxide) and purify the commercial nicotine with silica gel chromatography.