Sounds like you’re dealing with a lefty fork or headshock Cannondale? Tricky to find. Cannondale Experts are out of stock. So is eBay.
BTW, spacers don’t come in 4 mm increments so you would have to cut off a 4 mm section from a 10 mm spacer or, just use 15 mm total (10 + 5). If you’re lucky, you might find a 10 and 2 x 2 mm spacers. Better yet, a 20 mm spacer cut down to 14 mm.
Come to think of it, I’d look for a machinist who can make you one. Or someone who can layup a carbon one.