Search:

http://img.photobucket.com/albums/v185/ducksterman/Spokeswithtubes001_zpsd2d3b9a4.jpg

http://img.photobucket.com/albums/v185/ducksterman/Holdingtube001_zps32537992.jpg
...

That's where I got mine ...the cycle shop...not sure if now the more modern wheels are made differently...they might be scarce.
...

Allan, that's one of the reasons a bike spook works ...they have a thingy on the end which is like an adjustable nut that can be used to jam against the tube....some just have a bend on one end ....