You can do a search for your type of work by clicking the blue Lookup Business Codes option. And if you don't see anything that matches your line of work, just put in 999999, which is miscellaneous. It's not something the IRS needs nor will it have an affect on your return.
These are some examples along the lines of real estate, but it doesn't have to be exact:
Offices of Real Estate Agents and Brokers
"Agencies, real estate escrow Agents' offices, real estate escrow Consultants', real estate (except appraisers), offices Escrow agencies, real estate Fiduciaries', real estate, offices Listing services, real estate Real estate asset management services (except property management) Real estate consultants' (except agents, appraisers) offices Real estate escrow agencies Real estate escrow agents' offices Real estate fiduciaries' offices Real estate listing services"