If your university allows it then check facebook for those types of groups. Ask the admin permission to use the group for your data collection. SurveyMonkey is another resource that you might be able to use.

A huge amount of academic survey research using is being conducted on Amazon's Mechanical Turk. https://www.mturk.com/

According to Princeton University's Survey Research Center, "Amazon's Mechanical Turk service has become an increasingly popular way for researchers to conduct online experiments. Anyone with access to the Internet can use the service as a way to draw respondents for any type of web-based survey or experiment. It's a relatively low cost way to engage a diverse set of respondents in a short period of time." https://psrc.princeton.edu/our-services/using-mturk

As a matter of behavior, these people (stockbrokers) are not so easy to get them answer a questionnaire. Try some other methods.

I agree with my RG colleagues,

Dr. Emmanuel,

Dr. Scott Sampson

Dr. Michael Neubert

Dr. Laverne A. Davis

In their good responses.

Best regards

I think its better to contact the stock brokers to do survey on investors and collect the required information by using google forms and/or survey monkey. It may be the best way to gather information.

Use google forms / survey monkey / email/ linked in.