I don't know what the market is like but it's certainly a role, perhaps slightly more in the US which requires ASL interpreters for most publicly funded television as I recall. There are even hip-hop SL interpreters at concerts - Waka Flocka was featured in a viral video where he jumps down and dances with the interpreter and tries to follow the signs along with her. However, both those examples are ASL. In the UK, BSL doesn't seem to be as widely mandated and hence used, regrettably. However there will be at least some degree of work in creating e.g. pre-recorded videos of instructions or welcome packs for corporations and so on.
I believe, in general, most translators and interpreters (not just SL) work freelance, although often they'll be subcontracted by a dedicated translation third party service provider. I don't believe they're formal salaried employees though, and I'm quite certain they are normally contractors. I imagine it requires a deal of entrepreneurship and networking to properly make contacts as a freelancer and build up a core clientbase, and freelancers will probably do contract work as above to fill in gaps where they don't have any other work and to have evidence of their abilities in the recent past. I imagine pay will vary enormously depending on whether you're doing contract work for a company, freelance work and who you are working for in both cases. However as it's not going to typically be a salaried role I imagine progression will be a little limited - experience will only allow you to charge so much more before they'll look elsewhere - unless you involve yourself more in the business side and set up some kind of company or agency for interpreters and do less interpreting yourself and focus on hiring others to work while chasing contracts and so on.