The most suitable replacement for this is the
H2O International H2O-US4-13
In 2008 the NSA 100SX was permanently discontinued. Further, the NSA company has sold it's customer list and exited the water treatment industry altogether. Once the industry's largest brand, the NSA water filter product line is now orphaned.
The company NSA sold to offers the same line of H2O International products that we have been recommending as replacement upgrades for NSA filters for 13 years, but at much higher prices than ours.