You can try reaching out to Shopify Expert Developers or independent freelancers on platforms like Upwork or Freelancer. You can post a project description stating your requirement for adding a local payment feature for IDeal on your website and receive proposals from interested developers. You can then review their portfolios and ratings, communicate with them to see if they are a good fit for your project, and ultimately select one to work with. You should also clearly communicate your budget and timeline for the project to ensure that you find a developer who can deliver within your constraints.