A locked phone is generally a software lock on the phone to keep you from taking the subsidized phone and going to another carrier (thus losing them money). Unlocked phone just means you can use SIM cards from different carriers. Generally this is necessary if you want to use a SIM card in a different country. If your phone is locked and you want a temp phone, consider purchasing a used unlocked phone, or Amazon sells BLU branded phones which are inexpensive but should function perfectly fine as a travel phone.
If you have a locked phone and put in an incompatible SIM, it will display something like "SIM not compatible" and refuse to work.
Roam mobility is a USA/Mexico option that has popped up from time to time as a good and easy to use prepaid option.
https://roammobility.com
Quite a few users seem to mention it's a good inexpensive option. If you go frequently to the USA, the cost of the SIM card is negligible vs some of us that infrequently go.
I believe London Drugs sells this SIM. Monthly plans for US are between $40 - $65 CAD. Snow bird plans are even cheaper, but must be purchased in 90 day equivalents.
https://roammobility.com/monthly-plans