Not sure what options are near you, but check out any sort of convenience stores, drug stores/pharmacies, grocery stores - anything that sells any sort of gift cards. If you can't find Steam gift cards specifically, but have a way to get any sort of prepaid gift card that can be used online, those could work too. Not all prepaid cards can be used online though, so mind that.
Do your parents trust places like Amazon? Not sure if Amazon is in Trinidad, but if so, and if they'll trust Amazon, you can get Steam gift cards from there too I believe.
Hopefully you'll have a local store that sells gift cards. Best of luck with this!