I have a car that has a leaky evaporator coil, too. Here's what I have found:
A leaking evaporator coil is usually replaced. On most vehicles, this is a labor intensive job requiring complete dis-assembly of the dashboard to gain access to the coil. Your particular vehicle *may* be different.
Some companies offer products specifically made to seal relatively small leaks in the refrigerant system. Independent reviews that I have read show mixed results and some indicate use of some of these products can cause greater harm to the system. Some manufacturers claim their product along with a system recharge is all that needs to be done. Others require replacement of the filter and dryer so that the air and moisture is able to be completely eradicated from the system, this is because once system has been depressurized due to the leak, air and
moisture eventually enter. That air and moisture must be purged before running the
system because water can not be compressed and will create issues in the A/C compressor. It is important to note that some refrigerants are marketed with a dye to help locate leaks and even some with a "leak stopper". These products are fine to use. It's the stand-alone leak stopping products that I'm talking about here. You should try the refrigerants with a leak stopped / dye. They can seal small, slow leaks effectively. If you need to recharge annually - that may be the way to go. Large leaks that empty in as little as a couple weeks or a month will probably not be stopped with these products.
For these reasons, I have opted to spend money on replacement of my evaporator coil, rather than risk damaging the entire system with a "stop leak type product". Here's search result page
for "stop leak" products that you should review.