Take a look at this 2010 article discussing pancreatic cell lines (see Altex link below). Table 1 has a list of insulin-secreting cell lines (including your MIN6), discussing advantages and disadvantages of each. Most of these lines are derived from insulinomas of different species. Many labs will be willing to share their cell lines with you, but may require a material transfer agreement (MTA) to be signed first that dictates how you can use them (for example, for nonprofit research only). Contact labs that developed lines to see if they will share!
More recently, a very cool cell line called EndoC-βH1 has been developed,that seems to have many diabetes researchers excited. This human Beta-cell line responds to glucose, expresses GLUT-1 and GLUT-2, and secretes insulin upon stimulation with glucose in the physiological range (see Pubmed link below). If you think this line can help you out, a scientist (Dr. Ravassard) has previously offered to other scientists to give them these cells if they sign a MTA (see Researchgate link below).