Gymwear startup looking for manufactures

Dear all,

What is overal a good place to start if I want to create some gymwear with a specific design in mind?

Is it as simple as checking on alibaba for manufactures and let them alter excisting products? Or are there known manufacureres where you can send some design ideas and request samples?

I am also willing to fly to (for example) china to speak to suppliers and manufacturers, are there any known places to go to?

Kind regards!