The extended magazines available fit the CM.123 (1911) and CM.127 / CM.030 (Glock) guns; there are none specifically for the Desert Eagle.
That said, the AEPs are pretty generic and as far as I can tell, all use the same internal components - thus the 1911 and Glocks being compatible. It might be the case that the extended mags can be made fit the CM.121 too, if you're prepared to modify the magazine release detent. They're plastic so could be filed-and-filled fairly easily.
No guarantees, I've not heard of anyone doing it.