You won't find it without paying.
You can calculate it when some wholesalers send you a list of stones, with mention of the rap discount in a distinct column.
But again, the prices change every day.
If you buy from a trusted vendor here on Pricescope, you won't pay a lot above the wholesale price.
Rap is not generally useful to the consumer. It doesn't take into account cut grade or the exact appearance of inclusions. When a pro looks at a stone they assess it as "twice Rap" or "10% below Rap" and use the Rapaport price to price their stone.
If someone is offering you a diamond below Rap you can be sure it's overpriced and low quality.