As you are developing the gym bag, I assume you already have the expertise on the material selection, design elements, quality expectation.

For manufacturer selection, 3 things are important.
1- Quality. How do they control their quality in mass production? What is their general approach? To save the day or continuously improvement?
2- Pricing as usual.
3- If they are really know what they are doing? Are they only manufacturing products or also designing/developing? For how long are they doing the business and how capable are they? This will help you to speed up your project, they can help you to solve the problems from the beginning. In design stage, prototype, manufacturing. They will also help with material selection as they have long time experience.

Let me know if you have any further questions. I am happy to help.