D.L.
Hope these help
http://www.kids-birthday-party-guide.com/monster-truck-in... (looks like you have to sign up for their free newsletter to print the applications, but as long as it's free, right???) and there's also www.dltk-cards.com.
I like to do them this way too because it never fails that I need just one more if I buy the pre-packaged invites and I feel like I can personalize it a bit more. You could also use a similar format to one of those and use clipart if you can't find exactly what you're looking for.
Good Luck!