There's not a generic part that does what you want it to do here.
Thru-axles are part of the structure of a frame or fork. Even if it wouldn't really matter in most cases, making a generic aftermarket thru axle that steps down to a smaller-than-designed diameter like one can imagine would be asking for trouble.
When this kind of conversion is possible with off the shelf parts, it's typically done via hub-specific endcaps. They have to be hub-specific for the sealing elements to work. Good 12x100 hubs aren't necessarily convertible to 15, because to do so implies a design that's much bulkier than a road hub necessarily needs to be.
Specialized has made a lot of spare parts for their stuff over the years, and finding an amenable dealer to look through the catalog to make sure there's nothing available for your hub is probably the first step here, and one that nobody here can conclusively do for you unless they happen to be a dealer.
You can also pull an endcap and see whether it's one of the many 12x100 front hubs with a 15mm ID bearing such as 6902. I'm not totally sure of all the design reasons for this, but there are very few convertible hub designs that put the thru-axle in direct contact with the main bearings. There are a few exceptions I've seen to that pattern but it's pretty prevalent, so if they're 15mm ID it's a pretty good bet there aren't commercial endcaps that just do what you want. What would go wrong by boring out the ones you had were that the case is a separate question.