For clinchers you do better to get a tube with a longer valve stem (and it'll actually be cheaper). Just get a 50 or 63 mm valve stem, whatever you like in any correctly sized inner tube. Any decent shop will have them.
The problem with valve extenders is that you typically can't remove the stem on clincher tubes, so you can get an extender that screws over the current valve stem but is bulkier and doesn't let you tighten down the valve stem any longer. You can even on odd occasions have the valve stem locknut vibrate down and seat itself, at which point the tire is inflated and you can't deflate it. When that happens, you really don't have much choice other than to stick a needle in the tire and puncture it. Tufo makes the nicest valve extenders -- they seal well, are correctly dimensioned, and let you remove the valve stem and reinsert it at the top of the extender where it's accessible -- but they thread into the inside thread for the valve stem and if you can't remove the valve stem, there's no thread. If you do find some rare tubes with removable stems, get the Tufo extenders at www.worldclasscycles.com
. Available in both aluminum and steel and in a variety of different lengths.