I'm not entirely sure if you're asking about a simple sleeping bag liner or a Vapor Barrier Liner (VBL) for sleeping bags.... they're quite different.
If it's not cold, just about any sleeping bag liner would work. I have a fleece one from Wally-Land and it's pretty decent for what it is.
I use a VBL when I'm camping and it's cold outside (from ~+25F and colder). They protect my sleeping bag from the condensation and skin oils, etc., that come off my body while I sleep. The VBL prevents that condensation from freezing within the down insulation which helps maintain performance.
I've had several VBL's over the years and currently have 2 - one is a Western Mountaineering and the other is a CampMor brand. Both are great, but the CampMor is a good bit heavier as it is made from a heavier-duty fabric.