Sigma sells sodium butyrate of a grade suitable for cell experiments. You can filter the stock solution before use. 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3070119/

http://www.animalfeedscience.com/article/S0377-8401(06)00151-9/abstract

Thanks Nai-Kei,

 Do you know if butyrate es suitable for working with the IEC-18 cell line?

butyric acid or butyrate is not part of the regular medium formulation. 

Therefore, when added, it becomes either an extra supplement or a drug treatment itself (you will need a no butyrate group as control). 

the toxicity of the metabolite is not expected to be high, 

As much as 50 to 100 mM was used (see ref. below) in vivo, suggesting very low toxicity. 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3070119/

Best