Thanks all -- most of the letters sources listed so far are not available in the US, unfortunately. We don't seem to have the same train and bank supplies here!
DSquire -- Thanks! I must have been using the wrong search terms, because yours did indeed turn up more possibilities. I hadn't specifically searched for casting -- so maybe that was the problem.
But true casting pattern letters brings its own problems. Unfortunately Mifco wants $197 for 125 letters,

and Budget Casting Supplies wants $1.67/ letter or $40 for a full alphabet and only carries 1/2".
Contrast this with the UK model train plastic full alphabets available for 6 pounds or so. available in many small sizes.
I did find one interesting reference to having letters made up by rubber stamp makers, here:
http://www.artmetal.com/files/imported/project/TOC/proces/cast/letters.htmlBut still would like to find a source for inexpensive plastic letter alphabets in small sizes, or some additional methods for making them.