Well, first, Archive of Our Own is a great place to look. It has a well - organized tagging system, so you can search for the specific characters 'Violet' and 'Klaus' and then look for any fics with the 'lemon' tag if that's what you're interested in. Another option could be FanFiction.net, though it might not have as comprehensive a selection for this particular fanfic as AO3. You can also check out some niche fanfic communities related to the source material of Violet and Klaus, as they might have links or even host the fanfic themselves.
You can try searching on some popular fanfic websites such as Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, so there's a good chance you might find the 'violet and klaus lemon fanfic' there.