Owner Property Manager companies, Owner construction and maintenance companies, Angel investor, Crowdfunder, Real Estate Syndicator.
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
Every state has different rules and regulations. Typically were the line is blurred is if you actually engage in the showing, promoting , leasing or doing any work for the landlord that would require a license
If you are exclusively going to do marketing than do not call it a referral, but a simple marketing fee that would be due only if the marketing you launched ended in the unit being rented.
The minute you start executing services that would require a real estate license, such as drawing up contract, taking applications, showing units etc., then you are in RE Agent territory.
Keep it simple and keep it clear and draw a very strict line on what you offer and you should be ok.