fastest way is to buy them, but they are generic if you want good subs, you need to utilize a few options
1. Reddit but be careful where you post
2. Facebook - use an alternate account to post in groups etc
3. Reddit was awesome until the admins went crazy just check the rules first
4. Buy niche related subs, hard to find and expensive typically they sell large niches not targeted niches, most of the time I bet they are just generic or fake accounts.
Finally you can buy anything or do the hard work and do it legit and grow naturally maybe buy a 1-2k give yourself some rep and then build naturally. But takes time and make sure the niche is solid.
You can have on amazing video go viral and then add similar content to that video and then get another to go viral and the subs will come, 1 good viral video can lay a solid foundation and keep repeating. HOpe this helps