You need to use retrofit housings if you don't intend to rip down the ceiling or don't have access to the space above your ceiling.

Retrofit housings clip onto the ceiling board and depend on it for support.

I don't think you will be able to find retrofit housings that meet the requirements for wet locations.( mildly put)

Davidtay:

We have a fairly large attic and have already installed a number of can lights throughout the house. The electricians were able to install them without ripping out the ceiling. However, the electricians were gouging us on the cost of materials, which is why I want to purchase the housings and trims on my own... but I'd like to get the correct ones.

Thanks!

With access to the space above the ceiling and confidence clambering around in the attic space, I would get new construction cans rather than retrofit/ remodel cans.

The retrofit cans depend on the ceiling board for support, whereas the new construction cans use nailed in hangers (which could be slightly challenging with the ceiling already installed).