If you are located in a rural area that doesn't have an Allstate agent it is possible to become appointed with Allstate and purchase a book of business - and still be an independant.
Some independent agents do have Allstate books. Typically they were appointed in rural areas where it didn't make sense to have an exclusive contract. However, with Allstate's ownership of Encompass (the old CNA personal lines book), they are no longer appointing new independent agents to write Allstate biz - they are writing only Encompass business. As far as buying an Allstate book, you're taking a chance -- I imagine it would be up to the discretion of the sales director for your area. First, make sure Encompass writes in your state. Talk to the local sales rep and get a feel for their position there. It's certainly possible to buy an Allstate book, but be prepared for the possibility of a book transfer to Encompass.