I hate to break it to you, greencomval, but I don't think you're going to be able to find a replacement motherboard outside of buying another S8003 (perhaps broken in another way and replacing the bad parts with the good). Sites like this one, this one, and this one, are able to manufacture or distribute replicas of parts for many different phones. The reason I imagine it's so difficult to find replacement motherboards specifically is because the motherboard holds all of the phone-specific information, such as IMEI, all the software and code to run the phone, etc. That's not something that is easily replicable, especially considering IMEI technology and laws. Every phone has a unique identifier, and the motherboard holds that identifier (IMEI).

Another thing to consider is that even if the motherboard was available, and IMEI wasn't an issue, and you had the skills and tools to make the replacement quick and easy, the cost of the motherboard usually outweighs the value of the phone.