I've got a soft spot for late-night ghost stories, and 'Carmilla' is one I keep coming back to. If you want the original novella for free, my go-to is Project Gutenberg — they have the text (often included in the collection 'In a Glass Darkly'), downloadable as plain text, EPUB, or Kindle-friendly files. I like grabbing the EPUB and reading on my tablet with a warm lamp on; it keeps the gothic vibe intact.
If you prefer scanned pages or want to see the original Victorian typesetting, the Internet Archive has several editions you can borrow or download. For listening instead of reading, LibriVox offers public-domain audiobook readings, which are great for long commutes. Wikisource and ManyBooks are also reliable spots. A quick tip: search for "Sheridan Le Fanu 'Carmilla' 'In a Glass Darkly'" to find the most complete public-domain versions. Some annotated modern editions are paid, so if you want footnotes or scholarly intros, you might check your local library app like Libby/OverDrive instead. Enjoy the creeping atmosphere — it reads perfectly under a rainy window.