Meetups work well too, especially for B2B and niche products, since a lot of people going to meetups are going for the same reason you are; to show off their thing.
Disclaimer: I'm starting on this journey myself and these are the things I've found limited success with thus far.
Few Options: 1. Friends, Family, Colleagues etc. Although, I do not prefer this option, as friends and known colleagues tend to be sugar coat their feedback, and that is not something you want.
2. One of them right here: https://news.ycombinator.com/show
3. If you can showcase your product on KickStarter or other crowd funding - brilliant, as it also kind of proves people will be willing to pay for your product in future.
4. Meetup Groups etc, walk into these groups, if you find people who would fall into your target profile. Do not hesitate to politely ask, if they would like to have a look.
5. If you have a target list, offer them free access, with a concise email or some other medium.
tl;dr: Anywhere you can see right people, offer.
It’s probably worth adding https://betalist.com to this list as well.