The DGD-5005 filter is a very popular filter for this system. It is a 5 micron sediment filter.
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/DGD-5005.htm
The Pentek EP-BB filter is carbon filter that filters down to 5 microns and also reduces chlorine:
http://www.waterfilters.net/Pentek/wholehouse/Carbon_Filters/EP-BB.htm
Any of the filters on the following page will work if you need more options:
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/Wholehouse-10-Inch-Big-Blue.htm
THe OR-100 o-ring is the o-ring:
http://www.waterfilters.net/Pentek-Culligan-OR-100-O-Ring_p_0-189.html
Thank you,
WaterFilters.NET