Can this be bought separate to the ultimate edition of forza 6??
It was included with all digital editions, not just the ultimate edition. The Xbox store lists it as a bundle exclusive so it look like if you got the physical disk rather than the digital download you are out of luck on the 10 year car pack.
The physical disc also came with a code for the 10 year anniversary pack.
Correct, I can confirm I got the code for the 10-year anniversary car pack with my standard edition physical disc purchase.