Many of the people who have already answered are right on point.

Do what you're best at in your wheelhouse, and if not, consider people or businesses that are in your network and refer your existing client to someone you know, like, and trust.

For example, in Gold Star, we have a lot of insurance companies represented. Sometimes representative for "Insurance X" can't help the client. Fortunately, they know the representative for "Insurance Y" can. The customer is happy for the coverage and thankful that Rep X took care of them by putting their needs first and making the referral to  Rep Y.

If you need a larger network, look around for the company/companies that could provide the service you are asked for and don't handle. Get to know them and how they do business. If it matches, there may be a way for you to refer to them. Down the road, they might be able to refer to you should they bump into something they don't handle.

Best of success!