Originally from New York, I have been a high end custom cabinet maker in San Diego for over 30 years
First, if you can find what you are looking for, in a furniture store or warehouse or anywhere it will almost always be cheaper than having it custom made.
Furniture manufacturers make hundreds if not thousands of the same piece and buy their lumber by the trainload. It is impossible for any custom cabinet shop to compete with that type of volume production.
On the other hand, if you have found what you were looking for but it is the wrong size, You have come to the right place
In some cases, modification may be the answer. There are times when a given piece of furniture can be modified to fit your needs.
In the case of modification, it would be wise to check with a professional before purchasing the piece to be sure modification is cost effective.