This user found a GBA SP Case that fits a pair of joycons perfectly.

aliexpress link

Theres a handful of 3d printable options;

https://grabcad.com/library/joy-con-travel-case-1

https://www.printables.com/model/219975-nintendo-switch-joy-con-case-customizable-magnet-s

https://www.reddit.com/r/NintendoSwitch/comments/8h6txo/i_3d_printed_some_joycon_travel_cases/

Or you could make your own out of a hard case and some pluck and pull foam inserts. Example.

You dont need a pelican branded case, theres tons of cheaper ones out there.

https://www.amazon.com/PFC-Case-Pluck-Foam-15/dp/B07DPM3JSK

https://www.harborfreight.com/3800-weatherproof-protective-case-large-black-63927.html