Go to your local aquarium/tropical fish dealer and ask. They'll be able to list pretty much all the providers within a reasonable driving range.

Also lots of cities/towns have RO water providers that will sell anywhere from single gallons to thousands.

source: used to be a marine aquarium hobbyist and bought thousands of gallons of RO water to make seawater.

Some grocery stores and walmart's have RO water machines outside their stores also. However, if you choose to use those it would be good to invest in a tester as they do not always change the filters on schedule.

Any drug store will have distilled water