Insurance agent is no different from a realtor in terms of how people get to know them - referral. You can ask fellow realtors to refer you the best one they normally use. You can ask your clients. You can ask other professionals - doctors, accountants, lawyers.
I would suggest that you interview and choose 2-3 agents and refer business to them consistently. It will benefit you in the form of creating a professional relationship with 2-3 agents who would refer business to you (sometimes, not every month). Anf you get in agreement with those guys of being a part of the professional network. You ask them to introduce you to their centers of influence. And you do same thing to them.
You need more than one insurance guy to be save and to provide good service to your clients. Insurance agents and brokers are limited in which insurance companies they represent. In California Farmers Insurance does not take woodshake roof. AllState requires automatic gas shutoff to be installed, many companies will not insure more expensive properties. Whatever the limitation - you need to have a solution at the time of closing the escrow.
When interview agents ask them about limits of their binding authority. For example, Farmers gives agents binding authority up to $1,000,000 in replacement cost of the building (not purchase price, land in not included). It means that when you need it, the Certificate of Insurance is just a phone call away for properties in that category. What if we insure 10,000 sq.ft. mension with $2.5 mln in replacement cost? - it will take an inspection to get it approved, which might be up to 48 hours or much more sometimes.
Insurance agent is a resource in building your business. Treat the issue that way.
Another suggestion - get LOCAL people from that list
http://www.recamp.com/professionals/ Recamp is the program, which supplies insurance agents with outstanding REAL ESTATE follow up cards. They send it to your clients on your behalf. You sign the cards. They put the card and your business cards into the envelop, sign, stamp and mail. It comes to your client from you, not from the insurance guy. That way you get people doing follow up work for you at their expense to earn your referrals.