You can buy RO
water from most shops and mix your own water at home with the salt of your choice, you don't necessarily need the whole RO
There are two main types of salts out there, ones that mimic natural seawater and ones that enhance it.
The ones that mimic saltwater simply try to match the proportions of minerals found in natural seawater. Brightwell Aquatics does this. We used BW at the shop I was running and I hated it. It smelled like eggs, it burnt (the guy from BW even said one guy had to go to the hospital from chemical burns when he stuck his wet hand in a bucket of salt to scoop some more out), and the buckets were almost impossible to open, you HAD to use a tool (or two) of some kind.
The other types of salt try to enhance on natural seawater. This usually means they start with natural proportions but then try to improve it by adding more of the minerals shown to be needed or used in a reef tank (like calcium, alkalinity, magnesium, etc.). The best will have a lot of trace elements as well, not just the 'top 20'.
I use Tropic Marin, regular not the Pro Reef. Tropic Marin uses all pharmaceutical grade ingredients (so they are pure enough to give to a human and are extremely consistent in purity). The only weird thing is that their Pro Reef actually has less of certain things because they assume you will be supplementing it via dosers, calcium reactors, etc. They even say they should have switched the names. I always get the buckets so I can easily open and reseal them and you get a bucket in the end (great for any fishkeeper).
If salt gets wet either from actual water or humidity in the air (because it is kept in something less than air tight and clumps up) it has slightly dissolved and then redried. When this happens the salts and minerals crystalize differently than before and are no longer the same and will not be the same again even after you mix the salt up. Keep your salt air tight.