I noticed that you called them a "client", not an "employer". Being self-employed makes the situation quite different from that if you were under a contract of employment.
When you work for an employer you normally have a "job description" which defines what you do for them, but nearly always includes something to the effect of "ad-hoc" tasks, which is vague and can mean anything else relating to the job. Take for example a personal assistant, secretary, or an office junior - these roles are clearly administrative jobs and will mostly involve office work, and yet in many places of work will be asked to bring drinks to their boss throughout the day, or perhaps prepare drinks for visiting clients. They would accept it as part of the job.
However, many self-employed persons like yourself rely on the loyalty of clients for income and will do a certain amount of "entertaining" of clients. This could involve taking clients for dinner occasionally or buying them a gift at holidays. Many clients expect this, and consider these things as rewards for loyalty.
In my job, I enjoy a measure of protection because of the employment laws of the country in which I live, as well as the fact my large employer has a proper human resource policy and dedicated department. My managers cannot force me to do anything unreasonable because they are beheld to the same employment policy that I am. I would quite comfortably go to my manager and say something like: "The expectation of me to prepare drinks for my colleagues has to stop. I feel that I am being treated differently from my colleagues that have the same role as me because of my nationality. Can I please ask you to ensure that I am not asked to do this non-contractual task again?"
However, as you are self-employed and these are your clients, I think you need to ask yourself what the consequences would be if you stopped doing this for them, no matter how polite. Are you easily replaceable? Is this not "entertaining clients", as described above? (Incidentally, in my country "entertaining clients" is a tax-deductible expense! You could actually save a bit of money by keeping record of the costs of the ingredients!)
If you are certain that you want it to end, you have considered the possible consequences and you are not afraid them, then the most polite way I can think of would be to say something like:
I'm glad you enjoy my drink. Getting the ingredients to make it on such a regular basis has become very time-consuming and something of a chore for me. I will try to surprise you with it on special occasions but I may not be able to make it every two weeks as I have been doing.
This gives them a valid reason for the change, and does not rule out completely you ever doing it for them again in the future. It may even be more well received if you surprise them on occasions, as it will be more of a "special treat" than just something routine.