Where Can I Find Custom App Development Services?

Hey guys! I need a custom application solution for my business that users can access from anywhere on their smartphones. A proper UI/UX with trendy features and smooth functionality that allows us to monitor and optimize after launch is a must-have. For this project, I set a budget of $3,000, but I must pay 30% in advance and the rest after launch. So, if you have any references for the custom app development services in mind, feel free to share; I would appreciate your recommendations.