Purchasing medications outside of the country because they can be found cheaper elsewhere is illegal in the United States.
http://www.fda.gov/forindustry/impor...ct/default.htm
According to the Personal Importation Policy you can only get medications from another country legally if you meet these requirements:
•The drug must be unapproved and intended for use in a serious medical condition for which there is no effective treatment domestically available
•There must be no commercialization or promotion of the drug in the United States
•The drug cannot represent an unreasonable health risk to the patient
•The request must be accompanied by an affirmation that the drug is for the patient’s use only and by the name and address of the United States–licensed physician responsible for the patient’s treatment
•The request is generally for no more than a 3-month supply of the drug
Getting cheaper prescriptions from another country is illegal because, "FDA cannot assure that foreign-made versions of FDA-approved drugs have been properly manufactured, are safe and effective, and are exactly the same formulation as the FDA-approved versions."
http://www.fda.gov/downloads/Drugs/G.../UCM297909.pdf
Ask your GI about switching to another medication if you can't get on some sort of assistance program through the company that makes Pentasa.