You can specify "fallback" fonts in order of preference, so that if one is not present the next one is used. For a FONT tag, for example, you could do:
FONT FACE="'American Typewriter', Courier, 'Courier New'"
In CSS, the rule you want would be:
font-family: "American Typewriter", Courier, "Courier New", monospace;
In both cases, it works like this.</font><ol type="1">[*]<font size="1" face="Geneva, Verdana, Arial, sans-serif">If the user has American Typewriter, that's used.</font></li>[*]<font size="1" face="Geneva, Verdana, Arial, sans-serif">If they don't have that, but they have Courier, it's used instead.</font></li>[*]<font size="1" face="Geneva, Verdana, Arial, sans-serif">If they don't have either of those but they have Courier New, then that's displayed.</font></li>[*]<font size="1" face="Geneva, Verdana, Arial, sans-serif">(CSS rule only) If they don't have any of those fonts (an incredibly unlikely occurence but one that shouldn't be ruled out), the browser's default monospace font will be used.
</font></li>[/list=a]<font size="1" face="Geneva, Verdana, Arial, sans-serif">Note that American Typewriter is a default font on OSX, Courier is default on X and 9, and Courier New is the Windows equivalent. I listed them with Courier first because Courier New comes with IE, but to be frank Courier looks better, so this ordering will make the browser prefer it to Courier New. The browser's default monospace font (our last-resort fallback) is not likely to be ideal, but it'll be closer to what you intended than regular proportional fonts.