Here's what you can do. This is the form you need to file with the Labor Board:http://www.nlrb.gov/sites/default/files/documents/48/nlrbform501.pdf
You can figure out which region you are in here: https://www.nlrb.gov/ and submit by fax or mail.
And here's the text of the NLRA: https://www.nlrb.gov/national-labor-relations-act Sections 7 and 8 are the most important in your case. At minimum you can file a charge for section 8a3, the part that says it's illegal to be fired for organizing.
I've filed various ULPs for NLRA violations. So here's what to expect: The form is easy to fill out and usually the Board Agent will help you if you have questions and call. (In section k, you will put 8a3) After submitting your charge, you will have to give an affidavit about what happened and submit any proof you have. Any other witnesses you can provide will be extremely helpful. You will have to explicitly prove that you were terminated because of union activity (whether associated with a formal union or not). Your former employer will get notice of your charge right away.
NOTE: The Labor Board does not care how shitty your boss is. They only exist to uphold the NLRA. You need to prove you were fired for union activity only.
The Board Agent will determine whether or not there is merit to your claim. If there is, the Board Agent conducts a full investigation and becomes an advocate for you. If there is not merit, you can opt to drop the claim without any problems.
Regional offices vary widely on their efficiency and efficacy. The process is long and bureaucratic so don't put too much stock in it working well. Best case scenario, you could be reinstated and/or get lost wages. Even if it doesn't work out, you get to annoy your old employer and cost them some money in lawyers fees.
You mentioned elsewhere wage theft. Go to your state's Dept of Labor page to get/file a wage claim form. Interfaith Worker Justice programs do a lot of work around wage theft and can be helpful too.
I'm not a lawyer, but have been through this process with workers. You can do this all on your own without needing a lawyer or assistance from a labor union.