If the idea is unique then the basic search should be a solid place to start.
Otherwise you can share the concept idea here and there are several community authors and buyers who should be able to advise also.
I appreciate you will probably be less keen on sharing it as the risk of someone copying the idea but with all due respect:
describing a concept is not really enough for anyone to go on alone if they did want to try and use an idea
it’s pretty rare or unheard of for even the best authors to come up with something that is worth developing and simultaneously completely unique to here or not done before.
Bear in mind also that the “uniqueness” rejections are not so much about styles of sites, It’s more that some submissions were too close in design, execution or features to existing items and hence were rejected.
There are thousands of one page themes available - envato won’t now reject any more which are submitted, as long as they have a visual design, UX, functionality etc. That is bespoke and fits the design and doesn’t just look like an existing file with some different images, colours and text.