Lawyers are not likely to do any work for free especially in this economy.
This answer should not be construed as creating an attorney-client relationship, and is for informational purposes only.
Lawyers who are competent will not do this for compensation. Further, there may be ethical considerations that prevent a lawyer from taking stock or other such compensation. If your startup is truly "hot", then you should find proper financing---nothing is more important than protecting your IP rights and using competent, experienced legal counsel.
Also, many lawyers believe that it is a mistake to file provisional patent applications rather than utility applications----provisional applications sometimes give small companies a false sense of security about the ability to obtain IP protection at low cost----and if a provisional is drafted incorrectly or fails to disclose the full scope of your invention, you can face financial disaster. There is no substitute for a"hot new startup" for having an adequate budget for IP counsel---you want independent, objective legal advice---not some lawyer with an economic interest in your company who will be reluctant to tell you bad news.
This does not sound like a pro bono situation. You are launching a commercial endeavor. You can try to negotiate for payment-in-kind, some stock or other ownership interest in the start-up, or an interest in any proceeds the patent may later bring. While someone may offer a discount to help build a business relationship, it seems unlikely that someone will accept your proposal to work for "free"--to invest his or her time to prepare the patent application, yet have no right to share in its success (no investor would agree to that). Your current proposal sounds like you are asking for a favor. You need to offer something that makes business sense.
This answer is for informational purposes only. It is not intended as specific legal advice regarding your question. The answer could be different if all of the facts were known. This answer does not establish an attorney-client relationship.
If your startup is as "hot" as you say, you can typically find a start-up centric law firm (in your area Wilson Sonsini, Gunderson, Cooley, Fenwick, etc.) that will forego fees until your first round of financing. Still, you need to convince them that your idea is worth taking the delayed-payment risk and the more sophisticated firms have seen *a lot* of startups that found themselves very hot. Good luck.
Q: "We are a hot new startup about to launch a new mobile technology ... ."
R: Well that's mistake about a hundred and two. You CANNOT launch a "new mobile technology" without first evaluating the patent landscape to determine if your "new" [I bet it's not] technology infringes about a thousand already-existing patents. If we assume your "new mobile technology" involves smartphones then the thicket of patents in that technology space is so dense no one, yup no one, knows who really owns what technology. See http://gigaom.com/mobile/visualized-the-tangled-web-of-smartphone-patents/ and also query Google for "smartphone patents." If your new mobile technology is something else entirely then, rest assured, that other market is just as crowded with patented technology.
Step one when researching and designing a new technology [regardless of what it is] is to perform "due diligence" by locating and compiling the existing patent "prior art" in order to, among other things, (1) learn the technology, (2) learn what is no longer patent protected [and therefore free to use], (3) consider how to design around the technology that is currently-patented protected, (4) consider how to improve the currently-patented protected technology that will soon lose its patent protection, and (5) compile marketplace-important information about your competitor's technology.
BEFORE you start selling, or even advertising for sale or even discussing your "new mobile technology" with potential investors, manufacturers, marketing folks, etc. you need a "freedom to operate" opinion from a patent attorney with significant experience in that "new mobile technology." That opinion will cost you many, many thousands of dollars and may take a month or longer. And it may conclude that, no, you cannot sell your gizmo without one or many licenses. If you simply offer your gizmo for sale without getting a green [or yellow] light from your patent attorney first then real bad things will likely happen when you're contacted by one or more patent owners.
You need to discuss all of this, and more, with your own intellectual property attorney licensed to practice in your state.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.