One method is to use MUSHclient (downloadable from this web site for free). Make a "notepad" window, and then use the Edit menu -> ASCII Art.
You can browse for various font styles by choosing a font file (about 16 are supplied). There are also "smushing" styles which control the way the letters run together.
The result appears in the notepad window, which you can then copy and paste into your MUD's greeting file.