Realistically, we are all going virtual. Any firm that doesn't start leaning that way will eventually start to fade away, because it is becoming a mandatory adaptation. Same with the client base; they will inevitably be forced to use technology.
I believe the growth of virtual services is not any different than in-person. Referrals are the best way, other than acquiring a company (which is not always realistic or possible). My clients certainly appreciate that I still offer all options, but nearly all of them opt for virtual, whether that is communicating via e-mail or phone, or exchanging files via e-mail, fax, encrypted e-mail, or my encrypted portal (same platform as my encrypted e-mail).
I created a page on my website dedicated to virtual services. I am pushing it, and I make it known to clients--current and prospective. I would not consider it a niche, but it is an adaptation--it is different working with a client you never see vs. one you can see in person, especially on the billing side. I am more likely to require retainers with people I never see, unless I also have signatory rights on their bank accounts and an agreement that I can pay myself.