How do I demonstrate that my EB-5 funds are from a lawful source?

Stock reports, proof of where original money came from - inheritance, employment, etc.

This involves a much longer conversation/consultation than a one-line response here. But at its core, you are going to have to demonstrate where the funds you used to purchase the initial stocks were from, what accounts the funds were maintained in, whether any capital gains taxes applied/were paid for, and etc. And of course, it will be complicated by the fact that almost 3 decades have passed since the original transactions. You need a full consultation before diving headfirst into this.

For this source of funds strategy, I would recommend starting with a detailed source of funds declaration by the investor which outlines his/her background and professional work history (including practice of investing in the stock market using earned income). I would then supplement this declaration with as much objective documentation as possible which corroborates the source of funds declaration. I would also focus on income/investment proceeds which were earned more recently and work backwards from that point.

You show where the money came from to buy the stocks, then you show how much they paid for the stocks and calculate the current value differential. It is basic forensic accounting. Also, if they sold stocks, this would be declared on the tax returns so that is helpful.

Purchase reports, stock reports, tax returns, and more can be used.

Just provide whatever records are available for the source of those funds since they began.

You will need trading records. To the extent the brokerage firms cannot go that far back, you will need to find the broker handling your account to prepare an affidavit as to your investment history, objectives, net worth and source of funds. If you invested directly with the companies, you may be able to find records with them as to your status as a shareholder and when you became one.