With my resolution (1680x1050 if it matters), I can only see 21 saved costumes in the window. The last one coincidentally begins with the letter "S" as well, but I can pull down the scroll bar on the right of the list to see the rest.
Did you even save any costumes that you named with letters after "S"?
-and-
Does it let you use the scrollbar at all?
If the answer to both of those is yes, then check your costume directory to make sure the files are actually there:
..\City of Heroes\costumes\
If you are trying to access costumes that you saved on the test or beta servers, make sure you copy the costume files over to your live directory first.
It does let me scroll and I have several files saved that are alphabetically after "s". It just won't let me scroll after "s".
Is there a way to see my entire costume list? Right now I can only see part of it. I can't see anything past "s" alpabetically. Am I missing something obvious or do I have to delete or move costumes?