Sivan, I have access to the Virginia Tech libraries which includes their vet library (which I have been told by my friend in vet school there has very little information on fish). If you need me to look up a specific article, I will be happy to do it for you. Just PM me the title and journal name. I also have access to all of the articles published by the American Fisheries Society.
I know betta are a popular aquarium fish, but there isn't much research done on them because they aren't a big money-maker. Most research done on fish is for recreational and commercial fisheries because people want to catch big fish and eat lots of fish. This hobby isn't part of the mainstream ichthyology and fisheries research. The closest thing the American Fisheries Society has to an "aquarium journal" is the North American Journal of Aquaculture of which some of the research can be applied to our hobby. The aquarium hobby is barely on their radar.
If you were to cite things, I would cite magazines like Practical Fishkeeping and Tropical Fish Hobbyist. While they don't have much on bettas, I have found a care article on the PFK website. I will also cite reliable online sources such as Fishbase.org which is good for temp and hardness parameters. There is a pretty good list of sources at the bottom of my betta fish care article in my blog linked in my signature.
However, if you have access to a college library, you might be able to find some ichthyology textbooks. While these will have little references specifically to B. splendens, you will find a lot of information about the labyrinth organ and anabantids in general. You can also find general fish physiology information including how the gills work and why ammonia is detrimental to them.
Good luck with your research! Do let us know if you find something really interesting!