I need an ASA 104 certificate to bareboat for 1 week in Croatia next September.
In the future you will have more responses if you start your own thread.
Posting an unrelated question to an existing thread is the internet equivalent of sailing with your fenders down.
In this case since I started the thread and it is a short lived topic no harm is done. Sort of like leaving your fenders down for a day-sail.
Did you mention where you are located? I missed that.
To answer your question you have to deal with a school because an ASA instructor can not certify you without a schools backing.
Many schools are very small with one or two owner/instructors with whom you can make a deal. Just tell them what your qualifications are and what your needs are and I'll bet you they will make you an offer that makes sense.
They usually want to use their boat because of insurance. They are looking for a daily rate and about 4 people in a boat. If you can tag along for one day on a 3 person boat I would expect that would be the cheapest. An instructor on your own boat for a day or so might actually cost more.
Since you are asking for something out of the ordinary at a reduced price a total lack of arrogance will get you far. It would help if you looked at this from the ASA schools point of view and be as cooperative as possible. I'm not suggesting that you would be arrogant, it is just that when someone who is vastly overqualified like yourself is forced to jump through bureaucratic hoops that seem stupid it is sometimes hard to project the image that will get you what you want.
You also might want to ask your charter company if they require 101 and 103 also. I have not heard of anyone who skips classes but I have heard of people who test out of them.
I would recommend that you buy the book(s) and study.
If you haven't taken multiple choice tests in a while you will want to study.
Getting a passing mark on a multiple question test is a completely different skill than knowing how to sail. Both are required for the ASA certificate.