Best I can offer is to suggest making a few phone calls to local shops who offer gunsmithing services and see who's got what on hand. Most professional 'smiths who do upgrade/custom work on rifles seem to usually have at least a few around the shop.
Otherwise, there's always Brownell' or Midway USA. Service with either is fast and they'll likely have whatever size and type you need or want in stock. Installation can be a simple DIY if you've got some basic tools and a little patience.
