Guess what? You can ALWAYS reciprocate. You probably just aren't thinking broadly enough. I'll explain.
Depending on your specialty area(s) you may not be in front of the types of clients that someone who refers to you might want to be in front of. It's no matter.
Fact is, you probably KNOW people who know the types of clients your referral sources seek. Right?
All you need to do is connect people. They will take it from there. Especially if they are bankers, insurance or financial advisor types, and others who appreciate and "get" networking. You don't even have to pitch on their behalf, they'll do all that. Just put the people together. Take them to lunch or something and get the conversation started.
Cool thing is, you only need to make the introduction to another potential referral source to please someone who's referred someone to you. And you receive enormous appreciation from both parties!
So don't get stuck thinking your clients aren't right for someone who has been good to you so there's simply nothing you can do. Think deeper to determine who else you know and whom they serve.
And don't forget the power of introducing clients to other clients, either. Especially when it makes sense for them to do business together or benefit in some other way from knowing each other.
Clients are your primary referral source and they will both appreciate you thinking of them and will probably be happy to reciprocate.
When you connect people together, you have done something very valuable. This is the essence of social capital. Give it whirl!