I just want to write \Sha without ruining everything

for someone who wants to use just one or two cyrillic letters in math, in a computer modern setting, the old wncy fonts are still distributed in tex live as part of the amsfonts collection, in type 1 format. (but they don't require use of the amsfonts package.)

this is the recommendation in the ams author faq.

I want to use some cyrillic letters for math variables, but there isn't any LaTeX support for cyrillic in the AMSfonts packages. How do I do it?

The following four lines will load the upright cyrillic font and define \Sh to access the letter "Sha".

    \DeclareFontFamily{U}{wncy}{}
    \DeclareFontShape{U}{wncy}{m}{n}{<->wncyr10}{}
    \DeclareSymbolFont{mcy}{U}{wncy}{m}{n}
    \DeclareMathSymbol{\Sh}{\mathord}{mcy}{"58} 

The location of a cyrillic letter in the wncyr font can be found by looking at the chart in the AMSFonts User's Guide or at a font chart created by TeXing the file testfont.tex (included in every TeX system).

the user's guide can be accessed with texdoc amsfonts or texdoc amsfndoc.