Karlie here from the Shopify Support Team! It sounds like you're looking to hire a developer to create a custom app for you. Shopify has a marketplace that showcases different developers, designers, marketers and photographers who you can hire to help you with different aspects of your online store. If you're interested in hiring a Shopify Expert, you can check out all of the Shopify Experts available to create custom apps, here.
Before you choose a developer to hire, I would recommend following these tips:
When you're ready to request a quote from a Shopify Expert, you can select the 'Request Quote' option from the developers page in the Shopify Experts Marketplace. Another option would be to create a job and then let the marketplace match you with a Shopify Expert that best suits your needs. To do this, you can follow our guide here. We have another guide that can help you with the hiring process as well, be sure to check it out here.
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!