This is over budget but a terrific size and for sale by a PSer with a good eye for stones.
https://www.pricescope.com/community/threads/1-4ct-i-si1-in-dkj-custom-platinum-setting.252487/ She is offering to sell the stone alone.
A second idea is to contact ID Jewelry and tell them your budget and and that you are looking for a price scope quality stone. They are great with budgets.
Third idea, just slightly over budget, but a little smaller than you want, but great vendor:
You can also contact them and see if something is coming in in the near future that might be closer to your specifications.