Sounds like you want it to be in non italic Courier if the font is available and in an italic font if not. The way I would do it is to embed a non italic font in the epub (I don't think you can legally embed Courier though, you might have to find a similar font that you can embed), then use this CSS,
Code:
@font-face {
font-family: "somefont";
src: url(../Fonts/somefont.otf);
}
.somefont {
font-family: "somefont";
font-style: italic;
}
With this styling in your XHTML,
Code:
<span class="somefont">Text</span>
Then if the reader doesn't support the embedded font it displays in italics of the default font.