Often times you just cant, its a lot of banks policies to not sell the bags, seems they have contractual obligations to the coin rolling companies/fed. Some people are so into policy and bureaucratic process they just wont.
There are also some who are just lazy and dont want to deal with it because they have to fill out some paper work and reset the machine, apparently this is too much work for some.
there are some branch managers who kick ass and are more than happy to sell the bags. They want to get out of the coins as the more cash on hand they have the more they can loan out, use etc. They are the ones who recognize that it saves them money by not having to dispose of it with the fed, and it gets them more working capital, and builds a relationship with someone who has that much money to toss around. Maybe they will re mortgage, get a loan, buy a car or take business banking too them or buy a CD. These are the ones who realize that if you treat people with money well and treat everyone well you get further! There is no secret, its just asking the right things and catching them at the right time and mood.