What are the best hiring platforms to find qualified workers?

You might want to look at your job posts. These are marketing just as much as you making an ad for a job. This means it needs to be positive and you need to highlight what the person can achieve by working with you, not just the money and not just what you want. For A players, you need to get them excited to work for you. I've had luck on indeed once I changed up my job posting.