12 inch filter cartridges are very uncommon so they will be pretty expensive. It might be cheaper in the long run to stick with standard size filters.
Here is the 10 inch standard size housing that 99% of the industry uses:
http://www.waterfilters.net/culligan/wholehouse/housings/HF360.htm
Here are the replacements that work well for the system:
Sediment filter:
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/P5.htm
Carbon filter:
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/C-1.htm
Thank you,
WaterFilters.NET