Plain text means the old ASCII text standard, which has no copyright symbol.
TextEdit otherwise uses Rich Text which is a reduced Microsoft Word format.
If you want special symbols to be sent to people who have any type of computer, Adobe Acrobat PDF format can preserve symbols and formats if you use the Print dialog's Save to PDF function. Of course the other person either needs a Mac OS X machine, or an Adobe Acrobat Reader on their computer Alternatively, you can make a webpage that supports the copyright symbol with the HTML code of:
© or
©
Use a text editor to edit your HTML such as http://www.barebones.com/ Textwrangler.