There are a number of starting points depending on information that is not contained in your post.
Was it a corporation or LLC? Check with the Secretary of State for the registered agent/manager for service?:
http://www.ss.ca.gov/business/corp/corp_irc.htm
Was it a sole proprietorship with a registered fictitious business name?
If so, you don’t say which county. Check the county recorder to see who registered the name.
They had a business license in your county. Go look at the application and use the filing data.
If they also sold health products, they were licensed with the Board of Equalization. Get that information.
Check with the relevant state court and federal bankruptcy court to see if there is a filing or pending litigation.
If they required a professional license (and it sounds like it would), check with the State Dept. of Consumer Affairs.
Was it a franchise? Check with the franchisor.
They had a business license in your county. Go look at the application and use the filing data.
Did you get paid by check and still have a copy of one? Check the bank account endorsed on the back. If you sue in Small Claims, it gives you the right to subpoena bank records.
If there was an office, check with the owner/property manager to see who leased the office (they may also be owed money and be willing to help – and share cost).
While I think BBB is useless, you can check to see if there were ever complaints and who responded.
All of this can be done for free and mostly online but, as a novice, a good bet may be to use a commercial public records service, and you can find many online by using Google to look for public records + [state name]. That should give you all the data that you need in one place for about $25-40. I’m not going to endorse any particular one in a public forum, but the biggest all aggregate the same information, and the cost will save you time. It will also be capable of giving you more, like real property ownership and, perhaps, telephone number(s). A licensed Private Investigator would probable be willing to do that level of heavy listing for around $200 and suggest available assets for a few $100 more. Skip and asset tracing is part science and part art. This list could be much longer. You have to decide if you want to learn the skills or buy the output.
Using a third-party is effective. Relying on one can start to get comparatively expensive. Much can be done on your own, if you’re willing to put in the time and effort and learn. Believe it or not, your local public library may be a fertile source of information, and even the Chamber of Commerce.
An intelligent starting point, before spending much money and time, is to try to determine if there is a pending bankruptcy. If there is, it could obviate all the rest, but there is a wealth of information in the court file.