Good luck, I've had the same issue getting any similar parts or supplies from Felt over the past number of years. It's really frustrating.

I have heard that a low profile stem like the TriRig sigma will work on the headtube of the IA's but I have never tried it. It won't have the bolt holes to secure the nosecone top component, but you could get pretty creative with heavy duty velcro, double sided tape or some super glue and rare earth magnets.

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the world's still turning? >>>>>>> the world's still turning

I have the more expensive dagger stem you linked above, so yes can tell you it is the correct one and works well.
As the other person mentioned, you could fit a tririg bar/stem setup on there, the only issue is the two little nubs on top of the nose cone would be in the way, might have to grind those off not sure, and then attach cone with velcro as mentioned.
If you search the forum, someone did this, tririg, with their IA.

A while back I was in contact with a guy at Time Sport through Felt Europe who sent me a part for my B series direct. I think they've been sold on since then to Pierer, but the email address on their Europe website is the same one that I had contact through, info@feltbicycles.co.uk, and the guys were really helpful. Expect you've already tried this route, but worth a shot if not. Otherwise a tweet or Facebook message might get some attention. Best of luck!

I had the same problem with my ia14. Finally managed to use a longer standard stem but had to ask a special part from local felt dealer that goes between the headset bearings and the stem (they actually sent the part free of charge.. pretty awesome service). And the clamp height of the stem needed to be low enough. Not sure this would work in your case as there seems to be more integration in your ia compared to mine.

Edit. Link to the part:
https://www.tritownboise.com/...raditional-stem.html

i was having trouble getting a riser kit and top bracket for my IA16 aero bars...all summer, both directly through felt's website and my local felt dealer. but in the last week or so, it looks like they got their parts factory moved from colorado to california and the main parts person got relocated as well and they're set up for business. can't quite order directly from the website, but the person on the other end of the web form, will reply and let you know if they have the part in stock. so i would try to reach out to them again.