Nrich publish new problems every month. Why not try and get a solution published on their website?
There is a menu specifically for students. You can sign up for an Nrich student newsletter if you want to be notified of new developments on the site. If you have not tried Nrich problems before you may find their recommended starter problems good to try. You can search for problems by topic if you wish.
Signing up to Brilliant (including an easy option for sign in for Facebook users) will allow you to join an international community and get free weekly, personalised problems. Questions at various levels are available. (See this post for further information).