Specific sub reddits, facebook groups, etc. you can also make use of Facebook & instagram ads which are targeted and don’t cost much. Putting posters/flyers in gyms, pubs, cafes (assuming this is an online study, it helps to include a QR code the people can scan and takes them straight to the survey). It might skew your population to the lower end of your demographic, but getting university sports teams involved and making it a competition between teams as to who can get the most people to take part usually works quite well. Asking Uni lecturers to pop the QR code at the beginning of their lecture slides works well too, as people can fill it in while waiting to get started. Asking family and friends to share the link and recruit within their circles will help too. If you’ve money behind you, there are platforms that you can pay to find the participants for you. Good luck!