Buy them.
I just made a search for buy youtube suscribers and found 2,000 subscribers for 58$.

First, I'm not sure buying them is a good idea as that can easily be detected. Second, why 100 subscribers? What is the significance behind that number?

@travelin cat nothing special about the number but I just want to test with 100 subscribers. How to get them manually or with any tool

There really is no way to get them manually other then by creating compelling content that others want to view. You need many videos that people feel are either helpful or special in some form or another to have people want to see future offerings from your channel. There is no tool that can do this for you.

If there was a tool, you can be sure that YouTube would ban it shortly after discovering its existence.

Hi

any seo tips @travelin cat to get quick subscribers manually....

Creating compelling content is the best way to get people to subscribe. Beyond that, the only SEO tip I can recommend is to make sure that your videos all have a text transcript that you add to each uploaded video.

One beneficial way to increase subscribers is to add annotations into your videos with subscribe buttons. Often people don't even look at the subscribe button unless you direct them to it via annotations, script, or text.