For Property owners and investors I use Reonomy. It's costs about $399/month. Their phone numbers are about 90% accurate but like most softwares, not all properties have contact data. For those we research the secretary of state who the principal is and then get their contact on people finder.
for tenant rep prospecting, we by batched data from data axel by a specific industry we want to target and then we buy the owners data. Unfortunetly it is very expensive at $0.20/contact. We now have a datae base of over 100,000 tenants but we have had to pay about $20k for but at least we own the data and we can mass email market or give them to our inside sales agent to call to set up lease need analysis meetings.
In the end you can spend for all the data in the world but unless you are afectivley catagorizing it and managing in a CRM, it really wont mean much. That said, you should focus on building a database of your own that you dont have to rely on incorrect data sources like CoStar.
James Storey, CCIM