Whether the average untrained clerk in a big-box store is capable of making any keys that will actually work is a totally different question. That will vary from person to person . . . added to the difficulty of an adult with large hands working with a tiny key blank.
If you own any lock, there is no law that prohibits you from duplicating the key, period!
I don't know about those brands but some brands of locks have numbers on the keys. The number corresponds to the lock. As such, you can order that number key from the vendor. If its a big brand like masterlock, your local locksmith may even have some in stock.
If you mean a regular old trigger lock, I use combination trigger locks that you can set. They only run about $10.00-$12.00 each. The only keyed ones I have are cable locks for lever actions and they all use the same keys so I have a bunch.
Based on some of the gun locks I've used, they don't seem to do a wide variety of key patterns. By fumbling around for a few minutes, everybody managed to identify 3 or 4 that were keyed identically. That gives you all the back-up keys you could want, plus multiple locks.