
The radiant apex (and apex) mounts are always exclusive to crown crates as they're one of the main incentives for people to buy crates. They also have very low drop rates for the same reason - it means some people will keep on buying crates until they get the one/s they want or enough gems to buy it.
The only other option is seals of endeavour, which were added to comply with Microsoft's policy on loot boxes (which says anything in them will also be earnable in-game). It's possible but they're time gated and it will take a lot of time to get enough for a radiant apex mount, even if you don't buy anything else.