Bob
Your going to need to measure the length and diameter of the filters there care two common sizes 10 inch being one of them
Once you know the dimensions you can pick from many online or a Culligan store or even Home depot.
In a two canister system there are many combinations of filters you can use but usually you start with a one micron filter to remove the sediment and larger particulate so as not to plug up the finer filter down stream as quickly and follow that up with a carbon block type filter usually a half micron rating to remove water born micro organisms , chlorine , pesticides and other bad stuff.
There are specialized filters that remove other heavy metals such as magnesium but for general use just the two mentioned above are good for everyday use.
The filters won't do anything for hard water and typically the farther west you go the harder the water gets and then the only way to treat hard water is a water softener.
https://www.rvwaterfilterstore.com/filter-cartridgesHope this helps
Eric
