Aasimar

[NineShadowEyes]

Is buying the entire Volo's book the only way to get the Aasimar race?

[iotech]

It's also in the DMG

[LordEntrails]

The DMG version is, I believe, different than the Volo's version. Of course, you can create it yourself if you do not want to buy Volo's.

But yes, you can not buy single items from the expansion books, only from the PHB and MM.