My first internship I kind of landed by talking to a company representative at a career fair organized by my university and then mentionjng said guys name in my online application. Not sure how much impact that actually had but I guess it might have helped.

My second internship I found via linkedin. I just looked for companies there that had somilar positions or actually posted internship positions and then went to their website to apply and look for possible other open positions I could send a slightly altered letter from my first application to. If a company sounded interesting but didn't have any internships on offer I sometimes just sent them an unsolicited application. Honestly I don't remember how much feedback I got on those though.

Should be obvious but I also didn't limit myself to looking for companies close to where I lived at the time. In the end I had to move literally to the other end of the country.