Aftermarket boot liners are a good solution if you don't need to replace your boot. Here is a list of liner makers;
Zipfit at : http://www.zipfit.com/
More info at : http://www.skiingmag.com/skiing/outf...328983,00.html
I use a Thermoflex liner with good results. However, you need to carefully match the liner to the boot and the foot. The fit process requires a good bootfitter and attention to detail, just like buying a new boot.
What boot do you have? What size? What ski do you use? Where do you ski and on what runs? What is your wieght & height.
Aftermarket liners are not a low cost alternative to new boots. If you need new liners becouse the originals "packed out", you might consider a new better fitting boot first.
Hope that helps,