You could use the CC-10 water filter. It is the closest filter to the IR-10 in terms of flow rate and chlorine reduction.
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/CC-10.htm
The filter dimensions are 9.75" L x 2.875" W.
Most of the Instapure housing models can use this filter. However, there have been a few older Instapure housings that have not been able to use this filter because the bottom of the CC-10 filter is closed with only a slight indentation in the middle. The IR-10's middle is open and some of the old Instapure housings need that open middle of the filter. You will have to look at the bottom of your Instapure housing to see if you can use the CC-10 filter with only a small indentation in the bottom of the filter.
The best thing to do is to replace the entire Instapure housing with the HF-360 housing. This housing is the same size as your current housing and has 3/4" inlet/outlets. If you have any IR-10 filters left, you can use them in this housing or you can use the CC-10 filters in this housing.
http://www.waterfilters.net/culligan/wholehouse/housings/HF360.htm
The reason it is best to replace the housing is that there are no longer any replacement parts for the Instapure systems. There are no wrenches or replacement sumps if your housing develops a leak. We still have some 274300 o-rings left for the InstaPure housings, but we won't be able to get anymore once they are gone. The o-ring is an important piece to replace every couple of filter changes in order to seal the housing cap to the housing sump.
You could also get a larger system to install on your cold water line. The flow rate is better with larger systems and the filters have more capacity. Here are a couple options:
#10 Big Blue housing:
http://www.waterfilters.net/culligan/wholehouse/housings/HD950.htm
The GAC-BB filter is a good chlorine reduction filter for this system:
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/GAC-BB.htm
#20 Big Blue housing:
http://www.waterfilters.net/culligan/wholehouse/housings/20_BigBlue.htm
The GAC-20BB filter is a good chlorine reduction filter for this system:
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/GAC-20BB.htm
Thank you,
WaterFilters.NET