Try www.paperdirect.com
You may find some stock there for an invite you would like. Also you may consider doing the stamps and making your own card with embellishments from the craft store from the scrapbooking section.
You may want to consider doing small pirate chests with maplike note cards on them but this will run you over the $1.50 but would be a cute keepsake for the party.
Another great resource for party things which may give you great ideas for this party would be www.orientaltrading.com.
I usually do my own invites in Microsoft Publisher which is much more user friendly than Word and comes with templates for invitations. You just drop in pictures, dates, times, and other particulars for your specific party.