Full disclosure, that I work at www.upcounsel.com , but if you're looking for a vetted group of attorneys with experience in startups, I'd give us a shot. Basic gist is, you post your job to our site and you'll receive custom proposals from attorneys that are ready to work with you. Many of our proposals are fixed fee based but those that are not give you good idea of the costs associated before you select your attorney. You can then develop a relationship with that attorney if you have more corporate work in the future - if you'd like.
The other option, if you are not concerned with working with an attorney and want a more DIY approach, is to go to LegalZoom. Pros to this are that it might be slightly cheaper, but the cons are that you need to do more work and you are not talking to an attorney that you can get advice from.