You can't get code for the ECU, unless you can download it yourself. There are several people and businesses who have downloaded and disassembled the ECU code for the various different DSM cars, but they have not made the source code public.
While this might seem like an unfair thing to do, it is important to realize that some of these individuals spent years figuring out the DSM ECU, which is based on a proprietary microcontroller not generally available to the public. They had to figure out not only how to access the program code, but how to decipher it into instructions, and how the instructions operated the engine - all without any kind of documentation or support from anyone. This amount of effort is not to be taken lightly.
Also, many people have either built this specialized knowledge into a business, or was operating on the DSM ECU as part of a business venture. Private individuals are understandably reluctant to share their hard-earned work for nothing, and employees have legal and ethical responsibilities to their employers to not divulge trade secrets. Either way, they have little incentive to give away the information.
It is important to realize that this is very much like any business operates - on the strength of specialized and/or restricted information that they have gained through hard work. There is nothing stopping anyone from following in their footsteps, if they are willing to invest the necessary time (and money) . Most people would rather do other things, of course, which provides a foundation for commerce.
The Last Word: DSM code may be available from various websites or vendors. At least, the people I know didn't have a lot of problem getting it.