I would consult Martindale-Hubbell, the national directory of lawyers. You can access it on the web via Lawyers.com (the company's internet presence). You might also consider calling Chuck Michel, who represents CRPA (California Rifle & Pistol Assn.). He may know of someone who does this type of work, with an emphasis on NFA issues.
Bear in mind that in CA, getting NFA transfers approved is a pretty tricky business. Take a careful look at the applicable sections of the CA Penal Code. Unless you only plan to own AOWs (except pen guns, which are prohibited), you'll need both a Federal SOT and State DOJ Permit as a manufacturer or dealer (they are different permits). The SOT and state permits are not inexpensive, and DOJ will not issue permits to collectors (except possibly for DDs). Also, you may need multiple permits, depending on the type of weapons you handle. MGs is a separate permit, as are AWs. I believe SBSs and SBRs can go on a single permit. DDs are also, if I am not mistaken, a separate permit. I cannot remember if suppressors fall into one of the other categories.
You will either have to be a manufacturer (and they mean a real manufacturer with milling machines, CNCs, etc.) or a legitimate dealer (with sales to LE and military). Expect to have to show compliance with local ordinances and zoning, as well as have your facility inspected and approved by CA DOJ before ATF will approve your SOT and any transfers. CA DOJ expects that your facility will have appropriate security (reinforced steel doors and door frames, pick-resistant locks, monitored alarm system, and very secure safes or shipping containers with heavy locks). Roll-up doors should be double and/or have magnetic locks.
If you process the transfers into a corporation or trust, bear in mind that the transfers are not to you as an individual. In other states, if the corporation or trust were going out of business, you could Form 4 Transferrable MGs, etc. to yourself. However, in CA, SOT/permit holders cannot transfer MGs, SBRs, SBSs, AWs, etc. to an individual who is not law enforcement or active duty military using the weapon for agency work - so you will have to dispose of everything to another SOT.
In other states where MG ownership is available to civilians, operating as a sole proprietor has the advantage that you can keep the Transferrables and pre-May 1986 MGs if you give up your SOT, since you have already transferred them into your own inventory. However, this option is not available to Californians because of the restriction on individual citizens not owning most NFA firearms. As an individual, you'd have to continue to hold your SOT and State Permits in order to even possess most NFA firearms.
Note also that the purposes for which permits will be issued is quite limited. Your customers may consist of LE, military, movie industry and television - and other SOTs who provide services and arms to these customers. Manufacturers may also sell to other SOTs out-of-state.
Incidentally, setting up a trust will probably cost a good deal more than $600.00, especially with what you have in mind. A routine trust in San Diego costs between $1,500.00 and $3,000.00 if you have someone reputable do it. Since I charge $350.00 an hour for my family law, children's law, and regulatory work, I know how expensive legal work is. My firm's minimum retainer is $4,000.00 (and it can be significantly higher). Considering the consequences of a screw-up in formation of the trust (unlawful possession of MGs probably would net 10 years in State Prison, high capacity magazines - each mag being a separate felony count - and the cost of criminal defense cases), you'd be well advised to think through this carefully before undertaking this project.
If the purpose of your project is personal, then add up the cost of maintaining a legitimate business or trust to do this (expect it to exceed $2,000.00 a year at a minimum just for the privilege of operating an NFA dealership, without considering the cost of rent, alarm, time lost for the annual inspection by ATF, periodic inspections by CA DOJ, and depending on where you live, possibly local PD or SO inspections as well). This is not an activity for the faint-of-heart or those without substantial financial resources.