What would be the benefit of an alpaca/cotton blend? Unless there is some significant advantage over other batts, I would be leery of buying any wool batt that is not either Hobbs or Quilter's Dream -- which are manufactured specifically to not beard, shrink drastically, or distort with normal modern day washing methods. Before manufacturing of wool battings became more sophisticated, quilters had to completely encase a wool batt in cheesecloth before layering -- to retard bearding. There are still wool battings on the market that require this kind of treatment.
JoAnne Fabrics online carries Mountain Mist Fat Batt. Our local JoAnne's carries Mountain Mist batting; not sure if they have the Fat Batt, though. I have used Hobbs Poly-Down, which is a thick but soft polyester batting, and really like it. I think it would be a good alternative to MM Fat Batt.