First of all, there’s no such thing as “buying high quality followers”. All the followers bought are same, don’t trust anyone saying they will give you real or high-quality followers.
I think you should focus on growing organically as buying these followers will instantly drop your engagement and you won’t be receiving those brand deal if the identifies.
You can actually verify by auditing their profiles or their customers :P
Audit their followers with the tool Followeraudit and get to know how many of their followers are real and fake!