Typically with things like this you simply want to create a function that returns the element, so you end up with cleaner code. jQuery of course provides this kind of capability, but depending on your project you may not want to include the entire jQuery library only to use a small set of features/functions.
So the simplest version of what you want would look something like this:
function _($e) { return document.getElementById($e); }
_("a1").innerHTML = "Letter a";
_("a33").innerHTML = "Letter b";
_("cx").innerHTML = "Letter c";
_("dz").innerHTML = "Letter d";
_("x99").innerHTML = "Letter e";
_("fish").innerHTML = "Letter f";
But as I keep reading your post I wonder if you are actually asking for a way to reduce the number of lines, not just the size of the code. So instead of having 12+ lines of this same thing, you would like 3 or 4. But if this is the case I would need to know a little bit more about your code as there are several ways this can be achieved, however it will depend on how you have things set up in your code. For instance, you could use a for() loop to go through a list of elements, but this requires you to have those element ids and values you want in an array and I'm not sure if that's possible in your case or not. You could also use the .childNodes method to loop through all of the elements, but this only works if all of the elements are in the same container.