They have some differences. The anime might adapt the story in a way that emphasizes visual and auditory elements, while the light novel can offer more detailed inner thoughts and descriptions.
You can try looking on popular online bookstores like Amazon or Barnes & Noble. They usually have a wide selection of light novels, including Violet Evergarden.
Many local bookstores might have it in stock. You could also check with specialized comic and light novel stores. Another option is to search on the official publisher's website.