Web design and development firms in your area that you contract with as a freelance writer. SEO firms for that matter, too. However, the difference is you set the rates and as much as clients choose you, you also choose your clients. Actually, setting your own rates and procedures often "separates the men from the boys". You are always free to negotiate a lower rate than usual if you choose, but when you do make sure your reasons are sound reasons that will advance your business. Additionally, you are always able to thoroughly check them out before you contact them.
Depending on where you live, you might also scout around your neighborhood or your village for local small businesses. Make a list of them and look them up on the web. See what they have for a website or if they have one at all. If you think you can improve their content or write it from scratch let them know through a phone call or a cover letter that you are interested in helping them advance their businesses.
Online, make sure you build a web presence for yourself. Join groups like LinkedIn. If you know who your clients were at your last job, ask them for a referral. That's actually how to build your business. Referrals and testimonials mean a lot to a freelancer, but you generally won't get them unless you ask for them. Now, some people might disagree with me on this, but if you know where your work is on the web, I would contact the businesses and let them know that you are the person who wrote their content (unless you have something in a contract that explicitly forbids it). I wouldn't ask them for work, but I would ask them if the content is working out well for them and if they would be willing to give a testimonial or referral. I would be clear to them that you are beginning freelance and not contacting them as a representative of your former employer.
Start a blog or a hub page and write about what you know and about what you are passionate. If you have the money to invest, build a portfolio website.
If you are an English teacher, then you must have a good grasp of the mechanics. Start as an editor. Find things that could use improvement and propose to improve them. You will certainly have the background to show prospective clients.
As a freelance writer, your possibilities are endless. Aside from content for web pages, think newsletters, press releases, manuals, web interviews. You can even go beyond the web with brochures, direct mail sales letters, etc.