Any C Library sources for printf() and scanf() will get you working code samples that you can compile or manually convert to assembly.
Just type "sscanf.c" into Google and start searching. Here's a start:
ftp://ftp.update.uu.se/pub/pdp11/rt/11sp68/604/
There might be something here, although the sort of programming you're looking for is in all compiler libraries, so it is unlikely someone has written and posted anything like that:
http://www8.cs.umu.se/~isak/snippets/
Why aren't you using a compiler?
Tom