Specialist firms tend to exist to cover an area of expertise that other firms can't manage and while finding good front end developers can be hard this is because the market is saturated as anyone who can using strong tags considers themselves to be a UX person. This means you will end up competing with a large array of freelancers, full stack agencies, and design firms.
If you do plan to go down this route expect to be asked the question why? Why don't you have a backend development team, what benefit do you bring over a full service agency, what do you do that a full agency team can't?
I work for a small development agency, we are specialise in building payment systems its a unique selling point for us.
Why don't we have front end team? - We work with companies existing design teams to provide brand consistency and allowing us to keep to our core skill set. If the company doesn't have their own design team we have worked with several freelancers who have worked with us in the past and are use to our methodology.
What benefit do you bring over a full service agency - We bring knowledge not only in terms of coding in a secure way, but what legal issues surround payments as well as a bringing over best practices based on working only in this small niche.
What do you do that a full agency team can't - our team has a specialist members who are trained to perform PCI Audits, our system administrator formerly worked for a bank, as a company we have access to the legal information needed to setup and manage payment systems.
For us we have such a small niche that is a development, legal, best practice nightmare and made it our own we also banked on the fact everyone has to take money from each other. However we regularly lose out to full stack agencies and often it's because we don't have a designer or that the company wanted to build the project into a long term relationship.
Can you only offer client side services? Plenty of companies already do, providing you can prove you bring specialist skills most other agencies can't and they are skills people need.
But what is your unique selling point and what advantage is it to you, are you really specialising in a small niche or just leaving money on the table?