Many but not all bike shops offer some level of bike fitting service, but there are also independent bike fitting studios and bike fitting is also a service offered by some cycling coaches, physical therapists and sports medicine doctors.
The most popular way to find a bike fitter is to search on Google. If you are looking for a fitter with a broad experience base and high skill level, search the database at the International Bike Fitting Institute.
If you are looking for a fitter who uses a particular technology, there will usually be a list of users on that brands website. Eg, for Saddle Pressure Mapping take a look at gebioMized.