Usually pro bono work means that the lawyer doesn't receive anything in return, including sponsorship. You might be able to find a local attorney that would agree to do this for you either as pro bono work or in exchange for sponsorship. Good idea on your part. You may want to call attorneys in your local area and ask them directly.
Also keep in mind that to be able to deduct donations, your sponsors typically can't receive a benefit (i.e., a donation in exchange for advertising may not be considered a donation by the IRS because the donor is receiving a benefit--advertising). See IRS Pub 557, page 32. That's something to discuss with your attorney or accountant. Good luck.
Providing this general response does not create an attorney client relationship.
Another issue you have to look at is the distinction between a charitable organization and a business. If your charitable organization creates business income that income is taxable to the organization. You will need to analyze whether, for example, ticket or concession sales are business income if done by the tax-exempt entity. I highly recommend you start with the helpful IRS (yes, seems like an oxymoron but true in this case) webpage linked below.
You should be able to find a lawyer that will do this pro bono without any expectation of anything back. We work with over 250 exempt orgs as clients and we provide pro bono services to many other small exempt orgs.