if you mean buy then the question is-
are we talking about a custom bicycle as in your own choice of components that are all readily available or a custom frame to custom gemetry and everything?
if you mean make then the question is-
do you mean you want to go through with a professional what components you want that are all readily available and then they build it up, basically the same as above? or do you mean you want a course to build a bike from individual components your self? or a frame building course to build a custom frame your self?
the answer to most of the above questions would be a good LBS (Local Bike Shop)
i think OP is being some what unclear here?
Cheers Ed