Window sill slant bricks
There may be a brick that has that shape already, but I have not seen any. If there were, they would be custom ordered. To match your brick you have in place would even be a taller order. Bricks made in one time of year under the same name will differ in color and texture to a degree from brick of the same name from the same company, just made at a different time.

If color difference it not that big of a deal, you may want to go in a different direction. 

Cast stone sills come in a variety of colors, but typically mimic the color of Vermont or Indiana limestone. They can be tailor made to suit the space it occupies.

It looks like Glen Gery has a line of cast stone, perhaps ready to go. You may be able to order  stock long lengths and cut to fit. They are usually made to order that I am aware of.