I am building an online platform for commercial real estate brokers, lenders,and investors to connect at one place. I want to know where should I get data for the people in this field.
There are multiple organizations where you can farm these names, some paid some free services.
These are all sites where you will find all 3 of your prospective clients pretty active. Or you can start with brokerage sites and farm their online profiles
Marcus & Millichap
Investors I would advise you join sites like AngelList, Investr etc which have open platforms.
LinkedIn and Facebook ( Groups) along with Twitter to a certain extent is a good resource