Your best bet is to locate a high-end card/paper company and order thru them. Pretty much the same as you would with a wedding invitation, etc. Linen paper is always a nice choice with a simple, elegant design.
An example of the text might read:
You and your guest are cordially invited to attend a dinner party
given by <your name or company name>
Saturday, the Thirteenth of August
Two Thousand and Three
at Six o'clock in the Evening
Located at <insert place>
usually followed by a requested RSVP
You should include a self addressed, stamped envelope for the RSVP