In Animal Crossing: Pocket Camp, how can I get a lot of Cotton?

Best option is to link up a My Nintendo account to your game (which has an added benefit of providing a cloud save back-up, so you can switch between devices with minimal issues) & do a bunch of requests for your animal friends.

There's a reward system with the My Nintendo service, where you can earn Platinum Points weekly by completed a certain number of requests (at 10, 50 & 100 requests... for a total of 180 Platinum Points weekly for Animal Crossing: Pocket Camp) in the game, as well as completing missions in other Nintendo mobile games as well as other Nintendo services (as they will mix all Platinum Points together).

You can then redeem those Platinum Points for in-game items. Like 5,000 Bells for 100 Platinum Points OR 20 units of a common crafting material (like cotton) for 50 Platinum Points. You can either sign into My Nintendo website OR access the My Nintendo info in-game to obtain & redeem your points.

NOTICE: Platinum Points MUST be claimed within 30 days of them being awarded AND MUST be redeemed within 6 months. Missions that can be obtained multiple times (like weekly requests) before claiming the points within that 30-day window. Platinum Points that reach the 6 month mark will expire at the end of the month. The My Nintendo website will provide information on how many points will be expiring at the end of each month (over the next 6 months).

As a result, if you can pick up an extra 60-80 units of cotton per week (as you will have 30 Platinum Points roll over each week, hence the variance in redemption) in addition to whatever your campers will give you.

Beyond this, all you can do is try to strike up a deal with your friends or check the market boxes of others for any cotton they place on sale.

Hope this helps!

hire some slaves

Nope

You have to ask peter piper