In order to find the best sleeping back liners for you, start by finding out how much you can spare for this purchase. You can get these at different price points from under $20 to over $100.
The most expensive liners are usually made of silk that provides a luxurious feel while being lightweight as well. Thermolite can also cost a lot of money because of its advanced insulation. For budget buyers, consider microfiber or polyester.
You can find these in different shapes from rectangular to mummy. Choose the former if you move a lot during sleep and the latter if you want better heat retention. You should check the height and width as well to make sure that it fits.
Read the reviews to learn about real-life performance. Get feedback on durability and comfort so that know which ones to avoid.