Hi there i'm new to this forum too but thought I'd give u my opinion, by the sounds of it your best solution would be to mix yourself, maybe invest in a reverse osmosis system and use a container with a heater in to heat the water up to tank temp, You can use a powerhead or air pump to help stir and airrate the water.
Add some salt mix to the water. Follow the directions on the salt manufacturers packaging. If no directions are available, the general rule of thumb is 1/2cup of salt mix for every gallon of water or summat like that, check the salinity is between 1.021 and 1.025 and leave overnight. Bit of a ball ache but to do regular changes like you say it's half hour trip every time, tot it up over a year and you'd be able to buy the ro
unit twice over.
Plus you could always order your Salt in on the net so you'd never have to leave out for ya changes again. I'm not a saltwater owner but done tonnes of research as I not got a local shop in England uk either and saltwater setup is my next project! Research is always the key, hope u overcome your problem