I'm sure mine look like plastic spacers so pretty much any suitable sized washers should do. Unfortunately I don't have the car at present so can't tell you the size. At a pinch I'd use a bit of nitro fuel tubing cut to size myself.
The
manual doesn't give them a part number, but it looks like they come with the rear hub carriers which are out of stock.
What Velocity have you got? I'm currently running the 5.5L (blue - very nice motor) in my D4, and have previously owned the 5.5R (purple - brutal) and the 4.5R which was insane and would kill your belts in seconds!