Most retailers should be fine with you wanting to buy an item from them, even if they have to order it in. If they aren't interested in helping you get the item, then find another shop.
Some manufacturers will sell to the public but most won't. Smaller businesses are more likely to help you out compared to huge companies that aren't interested in selling you 1 or 2 small items.
If you can't find what you want, maybe check eBay or something like that, or visit a hardware and make your own out of wood or Perspex.