is there a truth serum drug where can it be bought ?

There is no such thing as a real "truth serum." Various short-acting
sedative drugs (such as sodium pentothal) are sometimes called "truth
serums," but all they really do is make a person sleepy and relaxed,
which may loosen the inhibitions.

Drugs used for this purpose have included ethyl alcohol, scopolamine,
and sodium thiopental (more commonly known as Sodium Pentothal)?all
sedatives that interfere particularly with judgement and use of common
sense. Alcohol is used for this purpose by many people.
These drugs (including alcohol), like anaesthetics, have a recovery
period. Some leave subjects with intense nausea, disorientation, lack
of balance, tiredness or other discomforting symptoms for a
considerable time after being used, an excess of any of them can cause
death.
These drugs serve only to relax people and assist them say things they
may not otherwise say (ever seen a drunk argument ?). Most often
people under the influence of such drugs mix fact & fantasy, and the
truth is not always the truth. (Ever heard a drunk person tell a story
?) So there is no real truth serum, as seen on TV & movies. It helps
however if the subject believes they are only able to tell the truth
when they have been given a certain drug - a kind of hypnotic effect.