Fortunately they are easy to come by for cheap used. Check your local thrift stores for good deals or online if you want something particular. Wool, cotton, nylon, and ramie are all good materials to see on the tag and don’t be afraid to rehab one with a few small holes in it, fixing the holes with darning is easy and they look better when they have been well loved.

Edit: here are some pics of a cardigan (not shawl collar but I love it anyway) that had a few holes mostly around the back of the neck that I fixed up with some multi-colored darning in gray and orange so it is easy to see. None of my other sweater repairs are this visible.