If you want to make an accurate representation of the movie prop, then you'll really want to get the rubber grips. If your parents won't agree to using the credit card, then contact Blast Tech via email. They might accept another form of payment (such as a money order), but you won't know until you ask. Be polite, explaine the situation, and see if they will be willing to work with you on your predicament.
If you want you want to make a representation of an E-11 blaster with out regard for the way the movie prop was built, then anything goes. It's my understanding that the "fins" around the barrel houseing are supposed to be heat sinks and not grips, so it would not make much sense to construct them out of rubber (They would need to a material with heat conduction properties, metal or some "Sci-Fi material") You might make them out of thick plastic (styrene or sintra). You could even make them out of strips of wood.
For what it's worth, windshield wipers was a good suggestion. Not an exact amtch, but readily available.
