Pharmaceutical grade nicotine is available on the open market. You just have to know how to get it, and order it prepared in PG or VG suspension.
The levels that most e-cig companies use to create e-liquids can range from 36mg per ML to 300mg per ML. You need special safety precautions to work with anything above 50mg per ml. 100mg per ML must be handled very carefully. Above 100mg per ML you are playing with your life if you do not know what you are doing or have do not have the proper equipment.
But yes, you can source nicotine. If you are looking for a DIY supply, there are a number of vendors that sell 100mg per ML or lower nic which is pharmaceutical grade.