Aw! Congratulations on finding the girl of your dreams! And what a fabulous place you've chosen for your proposal.
Unfortunately, I'm not aware of the ability to "rent a photographer" in any of the parks. Photographers are available to do private photo shoots on site at many of the resorts, but you are clearly looking for something different.
My suggestion would be to change your plan just a tad. Have you considered having dinner at the California Grill? The dining room offers an unmatched view of the Magic Kingdom fireworks. If you time your reservation just right, it's feasible that you could step onto the viewing deck at the close of your meal just in time to see magnificent colors explode in the sky. You could make the most of the moment by dropping to your knee and popping the question. Then again, the view is often times just as good from your table. If you chose to remain there, perhaps you could work something out with your server and he or she could take the photo for you.
Another romantic option would be to arrange for a dreamy dinner in in Cinderella castle. You wouldn't be able to see fireworks from there but I can vouch for it being an ideal setting for a proposal.
You might also consider calling the
Disney florist. They have a wide selection of engagement gifts available.
Sorry to be the bearer of bad news with regard to the photographer. Hopefully one of these suggestions will help to make your proposal a magical moment for the two of you.