Around here, it can be bought in every big box store or nursery. I have seen posts from the eastern parts of the country, however, where people were having trouble finding it. I don't know how acidic your soil is there. If it is naturally acidic, it may be harder to find. If it is alkaline like it is here, you should have no trouble finding some sort of soil acidifier. There is also a soil sulpher product that is supposed to be safer than aluminate sulphate.

Naturally acidic soils are common here on much of the west coast and aluminum sulfate is readily available at any nursery as well as all the box stores and other places that carry plant fertilizers and amendments.

I'd be cautious about using alum as a substitute - there are many formulations of alum and their chemical characteristics are not the same. Plus, it would be difficult to tell if you are applying enough or overapplying since it doesn't come with any directions for use in the garden. Agricultural sulfur is useful for acidifying soil but if you do not have aluminum present in sufficient levels, it will not help to turn hydrangeas blue - it the access to aluminum that makes hydrangeas blue.

I purchased it at Agway. Not sure if they are national, but they are a farm and gardening supply store (not a fancy type place).