You should understand that .NET and modern versions of Windows (NT-based) support Unicode, not "Extended ASCII" or anything. ASCII is supported as a subset of Unicode, but not "Extended ASCII". Please see:
There was a lot of confusion, which is reflected in Unicode.org mail archive, but in the Unicode standard "Extended ASCII" is barely mentioned as a historical attempt to use 8 bits to represent some limited international support. This thing was never standardized.
Practically, you need to look at Unicode reference pages and find out how to encode your characters. But you can also use the "Character Map" application (CHARMAP.EXE) which is bundled with every version of Windows and find these character there. After all, you can simply type them.
If you still cannot figure out how to work with your characters, you should create a code sample which is good to reproduce just the problem and explain what are you trying to do and why.