See my answer to the same question:

https://www.researchgate.net/post/Dear_collegues_please_advise_if_a_human_pancreatic_beta-cell_line_is_available_for_a_purchase

Hi Helle, 

Avoid using 1.1B4 cells they are a very poor beta-cell model. Other than that the ENDOCELLS that Amos refers to in his other answer are probably the best current available human beta-cell lines available for glucose-stimulated insulin secretion experiments. You will have to pay a lot of £££££ to get the MTA for these cells. Also worth note that they tend to grown very slowly like real beta-cells so plan ahead. The EndoBH-1 would be my preferred out of the 3 cell types ENDOCELLS offer. Note that you can also form pesudoislets out of these cells once they are growing well. 

Good luck. 

Best wishes

Jonny 

Dear Jonny

Could you elaborate on your experience as 1.1b4 as a poor beta-cell model?

I would grately appreciate this.

Best wishes

Helle