Perhaps there is a hobby that both of you love to do. You can use this 'theme' in your wedding invitations as well. If you both love to fish, you could use fishing line and lures for your invitations, or a hobby of wood_working could provide you with a way to expand your hobby into creating some special invitations. Regardless, if there is a hobby that the two of you share that helps to define you as a couple, don't be afraid to use it to your advantage and choose the best part of your hobby to direct your unique wedding invitation choices.
There are many places where you can find these budget wedding invitations, but mostly online. There are several online stores which have incredible discounts and many offers in what concerns their wedding invitations and that is because they want to sell their stuff and they also want to help out couples who have trouble gathering all the money they would need for a large and very expensive wedding.