Wheelskins.com is another option. Not as neatly finished as the OEM or FR500 wheels but a lot cheaper and many more options. Basic one piece covers are about $40 at some of the online resellers and they go up from there if you want two-tone or perforated.
Here's a Mustang wheel from their
gallery:
Another option is to have it refinished at somewhere that specializes in automotive interior restoration or customization. This may cost as much as the FR500 wheel (or even possibly the OEM wheels) but the FR500 only comes in charcoal so if you want tan, and you want an OEM finish, your choices are limited.
Or... if you're feeling adventurous and are handy, get you a piece of leather, cut the old cover off and use it as a pattern and make a new cover yourself.