Agencies in the competitive service are required by law and OPM regulation to post vacancies with OPM whenever they are seeking candidates from outside their own workforce for positions lasting more than 120 days. (Agency, in this context, means the parent agency -- i.e., Treasury, not the Internal Revenue Service.) These vacancies are posted on OPM's USAJOBS.
If you are interested in excepted service positions and do not find any posted on USAJOBS, you should contact the respective Federal agency directly. The U.S. Office of Personnel Management does not provide application forms or information on jobs in the excepted service.