I'm just taking a few guesses but perhaps craft places like Joanns , Hobby Lobby or Michaels would have them. I bought our extra large house numbers on Ebay but I can't say whether there are letters listed by someone or not. Worth a check into though.
I saw some large numbers that were about 5"-6" high and had a high outward projection (not flat) that would cast a very definite shadow making them more visible than a flat number. - Not cheap at about $5-$7 each but they had all the hardware included (screws and anchors). I saw them at a very complete "Do- It Best" licensed store this morning.
I only saw it in a old line traditional and now full service store that I shopped in about 2 locations and 25 years ago before they picked up a license with the present licensor/supplier.
They now have two aisles of bird seed and bird feeders and you can still get the old-fashioned floor sweeping compound and then get a hot dog freshly cooked on one of the grills they sell as you leave.