PIG is good containers, but there is shipping.
In Gardena is Rotational Molding. But the usually only have 300g to 8000g tanks. If you catch them at the right time they may have some smaller ones. But they usually don't last long.
The Ones from PIG are a food grade HPDE, they are much thicker and more rigid. THey have a lift off lid rather than a manhole cover. Would be worth researching a little first.
Also I think for the salt container its worth investing in a cone bottom tank with a stand. This plumbed correctly really mixes the salt super quickly.
Dave B