There are multiple organizations where you can farm these names, some paid some free services.

IREM
CCIM
CoStar
Loopnet

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

SVN
Marcus & Millichap
NAI
Remax

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.