Water filters normally consist of a sediment pre-filter and a carbon pre-filter. The standard sizes for the pre-filters are 2.5" in diameter and 9.75" long.
If your water filter is not a standard size, you probably have to go back to the dealer who sold it to you and pay high prices for frequent filter changes. If you want to convert your system so that it will handle our high-quality standard-size filters, it costs only $12 for each housing, $3 for the sediment filter, and $15 for the carbon pre-filter (plus $5 shipping).
With this new setup, you won't have to change your pre-filters for at least 3 years -- and you can either buy them from us (current price is $3 and $15, respectively) or from most water and hardware stores.