Finger monkeys, also called pocket monkeys or tiny lions, are fictitious animals. There is no way that a primate could evolve to become the size of a human finger (approx. Five cm long) as the complex digestive and circulatory organs systems and brains of primates are incompatible with the high metabolic rates of such a small creature. Extremely premature monkeys born that size would neither survive long, or look like "finger monkeys" as seen on the internet. There are a number of images on the internet of monkeys the size of a human finger, however these are edited. There are no known primates that size. Some people believe that finger monkeys are pygmy marmosets, the species native to South America, specifically western Brazil, Peru, Bolivia and Columbia. However, pygmy marmosets, Cebuella C. Pygmaea grow to have a body size of 14 to 16 centimetres, much larger than a human finger. Pygmy marmosets are a vulnerable species, which means they are close to being endangered but not quite. It is illegal to buy, sell, or trade them.
Even if "finger monkeys" did exist, there would be strict regulations governing their sale. In the United Kingdom and USA the sale of any animal requires a licence, and different sorts of licence are required depending on the animal being sold. Here is a source showing pictures of finger monkeys, which is how the myth of their existence was started. These photographs are faked: damncoolpics.blogspot.com/2010/12/finger-monkeys.html
Also here is the source containing the information about pygmy marmosets: en.wikipedia.org/wiki/Pygmy_marmoset
Even if "finger monkeys" did exist, there would be strict regulations governing their sale. In the United Kingdom and USA the sale of any animal requires a licence, and different sorts of licence are required depending on the animal being sold. Here is a source showing pictures of finger monkeys, which is how the myth of their existence was started. These photographs are faked: damncoolpics.blogspot.com/2010/12/finger-monkeys.html
Also here is the source containing the information about pygmy marmosets: en.wikipedia.org/wiki/Pygmy_marmoset