Look at it like a 'project', first make your case clear including What is it that you want to achieve, for what purpose is the medresa, how many will benefit from it, how much do you need, how long will it take and so on. Then there are various ways to find a sponsor. You can prepare a proposal including all the above and dissiminate it among NGOs and well known wealthy people or you can contact social media campaigners and influencers and persuade them to assist.