This is not a good place to economize. All the good firms are expensive. But they will also do deferred fee deals with sufficiently promising startups. To get one, get an intro from a founder or investor they know.
I made a round of 20 good firm inquiries and no one defers. It is difficult to get an intro when you are from another country, although I have a potential patent to go along my business plan. Thank you for the feedback! I am posting first time, although I read HN almost daily and I didn't realize how difficult it is to get some traction on a question!