My path to writing textbooks and resources really began with this website, so I would strongly recommend sharing resources online in some form to get your name known/ prove your value, if you wish to build a career in textbook writing. Here are some ideas for places to start or make approaches, although I will say that a lot of my work has come to me through word of mouth or via other connections. Also: being an examiner is a great way to build connections and expertise.
Submit to Teachit, bearing in mind that they’re looking above all for classroom-ready materials which don’t require explanation/contextualisation and which fit the latest specs. Material you’ve used in your own classroom with your own students often needs adaptation in order to be suitable for other teachers to use.
Approach Emagazine, Mediamagazine, the English Review or other similar magazines with ideas for articles you could offer. Do not write an article without discussing it with them first, as you do not know what they’ve got ready for the next issue. The best way to judge how to write for these (or any) outlets is to read and analyse their contents for tone, address sentence length etc.
If you have a blog/website, always cite this as a writing sample as well as sending any resources or articles you have produced/had published.
You can approach publishers directly with a CV and writing samples as well as a clear statement of what you are available for writing.