I need to replace the spring-loaded lighting cover for the light in the tub
Trim kits for "can lights" (recessed lights) are sold separately.

The box stores have them or you can find them online. Like this one here.

For a shower/tub enclosure you need a trim kit rated for wet locations. It will have the gasket and springs like the old one.

Take the old one to the store with you to make sure you get the right size. They come in 4, 5, and 6" sizes. Or measure the "can" part of the fixture if you wish to buy online.

Good luck!