IBS is co-morbid (70-85%) with Chronic Fatigue Syndrome and often become other forms of inflammatory bowel disease (IBD) over time.
“Patients with inflammatory bowel disease have lower numbers of Butyricicoccus bacteria in their stools. Administration of B pullicaecorum attenuates TNBS-induced colitis in rats and supernatant of B pullicaecorum cultures strengthens the epithelial barrier function by increasing the TER.” 
This species of bacteria is named because it produces butyric acid. This acid is significant:
“Several recent studies have identified butyric acid as a potential therapeutic agent for IBD. Some gut bacteria produce butyric acid naturally in the intestines, but in IBD patients some of these strains are heavily depleted. Trials in mice have shown that injecting one such strain Faecalibacterium prausnitzii into the digestive tract is effective at restoring normal levels of gut bacteria and treating the symptoms of IBD. In addition, novel identified butyrate-producing strains, such as Butyricicoccus pullicaecorum, have been shown to exert similar effects.” 
This leads logically into two directions:
- Bacteria that produces butyric acid
- Supplementing with butyric acid
Bacteria producing butyric acid
The species cited above is just one bacteria (which does not appear to be available as a commercial product at the moment). There appear to be a group working on producing Butyricicoccus pullicaecorum as a probiotic according to this presentation and a related patent application assigned to the University of Gent, Belgium (any Belgium readers may wish to make some phone calls… )
The known species producing butyric acid are:
- Faecalibacterium prausnitzii ( low in IBD , 
- Butyricicoccus pullicaecorum
Clostridium butyricum    – is easily available in Japan. An application to sell it in the UK was made in 2012. It is available OTC in Japan (so if you have friend going there — tell them what you want to have brought back!). None of the online shops that I could find will ship to the US (Amazon.co.jp lists it – but will not ship it).
- Summary of studies here.
Butyric Acid Supplements
This is available as a supplement (GABA), for example on Amazon. There are many articles proposing it for treatment of FM, CFS, etc. for example, this article. Another form, sodium butyrate, is also available as a supplement and a 2011 study found
“In conclusion, our results show that oral administration of sodium butyrate improves mucosa lesion and attenuates the inflammatory profile of intestinal mucosa, local draining lymph nodes and Peyer’s patches of DSS-induced UC. Our results also highlight the potential use of butyrate supplements as adjuvant in UC treatment.”