I use swivel clips (aka dog leash clips) to securely lock the spring-loaded doors. A safety pin will work too, as long as your birds don't figure out how to open it. Most birds never do, but I once had a chick that figured it out and she went around unlocking all the cage doors just for fun.
I like to use swivel clips on some cage doors and safety pins on others. With some cage doors, one particular type works better than the other. I like the swivel clips on the spring-loaded doors because they don't let the door have as much range of motion as the safety pins do.
You can buy these clips on Ebay and other places. For example:
https://www.ebay.com/itm/5pcs-Metal-Spr ... 2154934468 They come in a variety of sizes, so check the measurements to make sure it's a suitable size. Hardware stores like Home Depot will probably have them too, but they might be cheaper online.